This PR changes the approach we're using to render the channel buttons
to use a more straightforward (and less hacky) approach.

### Motivation

Even with the variety of hacks that were employed to make the current
approach work, there are still a number of issues with the current
solution:

- Hovering in the empty space to the left of a channel doesn't correctly
apply the hover background to the container for the channel buttons
- Hovering to the very right of the collab panel (right on top of the
drag handle) causes the channel button container to apply its hover
background without applying it to the rest of the row
- The buttons would still get pushed off to the right by the indent
space in the channel tree, resulting in jagged indicators at small sizes

Additionally, the rectangular background placed behind the channel
buttons still didn't look great when it overlapped with the channel
names.

### Explanation

For these reasons, I decided to explore a simpler approach that
addresses these issues, albeit with some tradeoffs that I think are
acceptable.

We now render the absolutely-positioned button container as a sibling
element of the `ListItem`. This is to avoid issues with the container
getting pushed around based on the contents of the `ListItem` rather
than staying absolutely positioned at the end of the row.

We also have gotten rid of the background for the button container, and
now rely on the background of the individual `IconButton`s to occlude
the channel name behind them when the two are overlapping.

Here are some examples of the new UI in various configurations:

#### When the channel entry is hovered

<img width="270" alt="Screenshot 2024-01-09 at 6 15 24 PM"
src="https://github.com/zed-industries/zed/assets/1486634/0207a129-30eb-4067-8490-3b6c9cbf75ea">

#### Overlapping with the channel name

<img width="229" alt="Screenshot 2024-01-09 at 6 15 43 PM"
src="https://github.com/zed-industries/zed/assets/1486634/0a67507e-45c8-4325-a71e-d416edc3a115">

#### Narrow collab panel

<img width="116" alt="Screenshot 2024-01-09 at 6 16 07 PM"
src="https://github.com/zed-industries/zed/assets/1486634/887ebaa3-e4d6-4497-9141-afcc8b7cd356">

### Tradeoffs

The new approach comes with the following tradeoffs that I am currently
aware of:

The occlusion can look a little weird when the icons are in the middle
of a channel name (as opposed to fully occluding the end of the channel
name):
<img width="190" alt="Screenshot 2024-01-09 at 6 24 32 PM"
src="https://github.com/zed-industries/zed/assets/1486634/0062b806-1b8f-47eb-af8d-f061d9829366">

Hovering one of the icons causes the icon to be hovered instead of the
row:

<img width="232" alt="Screenshot 2024-01-09 at 6 31 38 PM"
src="https://github.com/zed-industries/zed/assets/1486634/cbcc1a45-64d2-4890-8ad6-c5a5ee961b86">

Release Notes:

- Improved the way channel buttons are displayed.

There were a ton of doc tests that weren't compiling in the `ui` crate,
so this PR fixes them.

Release Notes:

- N/A

We had introduced these mutexes at a time when `gpui2` required app
entities to be `Sync`, but they are no longer needed. Removing them now
because we're trying to find out why we're sometimes getting crashes and
deadlocks in livekit, and we didn't before.

If the remote sends us an invalid scroll position, we should not panic.

Release Notes:

- Fix a panic receiving scroll positions out of order

Release Notes:

- Fixed buffer search being dismissed not after the modals are

Co-authored-by: Piotr Osiewicz <piotr@zed.dev>

This PR renames the `Shape` enum to `AvatarShape`, since it seems pretty
specific to `Avatar`s.

Release Notes:

- N/A

This PR cleans up a number of references in doc comments in the `ui` and
`theme` crates so that `rustdoc` will link and display them correctly.

Release Notes:

- N/A

Add a test to catch regressions to
https://github.com/zed-industries/zed/pull/3964

Thanks for the idea @ConradIrwin 

TODO:

- [ ] Fix immediate stack overflow in test

Release Notes:

- N/A

Co-authored-by: Conrad <conrad@zed.dev>

Documents a number of items in the `theme` and `ui` crates.

Release Notes:

- N/A

Release Notes:

- Fixed an issue where all but the last scroll event would be dropped if
there were multiple within a single frame.

This PR cleans up a number of references in doc comments so that
`rustdoc` will link and display them correctly.

Release Notes:

- N/A

Co-Authored-By: Antonio Scandurra <antonio@zed.dev>

🍐d with @conrad @mikayla-maki on covering some of the surface area.

Release Notes:

- N/A

Co-authored-by: Conrad <conrad@zed.dev>

Co-Authored-By: Antonio Scandurra <me@as-cii.com>

Co-Authored-By: Conrad <conrad@zed.dev>

Co-Authored-By: Antonio Scandurra <me@as-cii.com>

Co-Authored-By: Conrad <conrad@zed.dev>
Co-Authored-By: Mikayla <mikayla@zed.dev>

Co-Authored-By: Antonio Scandurra <me@as-cii.com>

Co-Authored-By: Antonio Scandurra <me@as-cii.com>

The test in particular should be included in our release binary at
present. :/
Release Notes:

- N/A

This PR removes the `Default` impl for `StatusColors`.

Since we need default light and dark variants for `StatusColors`, we
can't use a single `Default` impl.

Release Notes:

- N/A

Fixes https://github.com/zed-industries/community/issues/2388
Release Notes:

- Fixed tab content of project search overflowing the tab for queries
with newlines.

This PR adds a `SharedUrl` type to GPUI.

It's just like a `SharedString`, but for denoting that the contained
value is a URL.

Mainlined from @nathansobo's GPUI blog post:
https://github.com/zed-industries/zed/pull/3968/files#diff-7ee75937e2daf7dd53f71b17698d8bd6d46993d06928d411781b9bd739b5f231R9-R12

Release Notes:

- N/A

This PR renames the `IconElement` component to just `Icon`.

This better matches the rest of our components, as `IconElement` was the
only one using this naming convention.

The `Icon` enum has been renamed to `IconName` to free up the name.

I was trying to come up with a way that would allow rendering an
`Icon::Zed` directly (and thus make the `IconElement` a hidden part of
the API), but I couldn't come up with a way to do this cleanly.

Release Notes:

- N/A

Release Notes:

- N/A

Release Notes:

- N/A

I think this will be a key user experience driver, but we do need to
find a way to enable it without widening our vector clocks.