Loading threads with server-side assistance (#2735)
* Fix bug where undefined vs null in pagination tokens wasn't correctly handled
* Fix bug where thread list results were sorted incorrectly
* Allow removing the relationship of an event to a thread
* Implement feature detection for new threads MSCs and specs
* Prefix dir parameter for threads pagination if necessary
* Make threads conform to the same timeline APIs as any other timeline
* Extract thread timeline loading out of thread class
* fix thread roots not being updated correctly
* fix jumping to events by link
* implement new thread timeline loading
* Fix fetchRoomEvent incorrect return type
Co-authored-by: Germain <germains@element.io>
Co-authored-by: Germain <germain@souquet.com>
