Threads, Thumbnails, Unified Push

* Thread UI, see demo: https://kumi.tube/w/1LQQp5Uia4u8Pdojxen1y8
* Blurhash, data URI, and bits-of-binary thumbnail support
* Extended addressing ofrom support (group text senders)
* Fallback indication 0.2.0 (https://github.com/xsf/xeps/pull/1188)
* Experimental Unified Push distributor from
upstream (https://gultsch.social/@daniel/109630522775489629)
* Fix encoding XMPP URIs when sharing
* Add gateway to roster automatically after registering with it
* Fix crash on devices that have no phone UI
* SIMS parsing (XEP-0385)
* Do not re-download a file if we have it already (by hash, using SIMS)
* Allow Conversations-compatible backup generation
* Confirm before message retraction
* Don't use delay time for sorting conversation list
* Support for xmpp:?command URIs
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