WebXDC, Conversation notifications, Inverted onboarding

* WebXDC prototype
* Conversation-style notifications, for per-conversation priority and sound
* Improvements to dialler integration audio routing
* @here triggers XEP-0224, which notifies as if mentioned
* Phone number and date formatting in Command UI
* Allow message correction to affect subject and thread
* New "Cleanup Conversations" option to save RAM for busy users
* "Inverted" onboarding where people have the option to set up a phone number
provider before Jabber ID if the provider supports that
* Try to get list of commands from any domain-only Jabber ID, without presence
* Welcome screen intro slides
* Restore backup prominent on welcome screen
* Do not ban registering with short usernames
* Menu item to add conversation to homescreen
* Allow manual command feature discovery for Jabber ID with no presence
* Follow XMPP URI provided by Command UI
* Check for FEATURE_CONNECTION_SERVICE on 33+ as well
* Fix many crashes
* Long press to copy link more reliable
* App privacy policy
* License check code for eventual Play Store release
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