Visual tweaks, OMEMO fixes, subject, local mute

* Allow locally muting channel participants
* Allow setting subject on messages
* Subject-only message sets thread subject/topic/name
* Display list of recent threads
* Support full MUC configuration form for owners
* Register with channel when joining, deregister when leaving (where supported)
* Fix sending files again from Cheogram's own downloaded files
* Slightly rounder corners
* Flatter chat bubbles
* Somewhat softer colours
* Default send button to attach icon
* Stronger default account colours
* Center buttons in chat bubbles
* Phone icon for "PSTN" type gateway
* Allow rejecting a contact request without blocking
* Always show presence name if known (but tag with jid for anti-spoof)
* Fix sending location with OMEMO
* Fix for saving first bookmark on server with PEP but no bookmarks2
* Process MUC password if present in XMPP URI
* Don't show aesgcm URLs to people
* Basic support for list-multi in Command UI
* Slider widget for Command UI
* Button grid even with Back action for Command UI
* Fix height for long, wrapped values in result forms (Command UI)
* Expert setting to choose voice message codec
* Fix import of backup with WebXDC updates
* Clear sensitive payloads when using legacy e2ee mechanisms