sliding sync: add lazy-loading member support (#9530)
* sliding sync: add lazy-loading member support
Also swap to `$ME` constants when referring to our own member event.
* Hook into existing LL logic when showing the MemberList
* Linting
* Use consts in js sdk not react sdk
* Add jest tests
* linting
* Store the room in the test
* Fix up getRoom impl
* Add MemberListStore
* Use the right context in MemberList tests
* Fix RightPanel-test
* Always return members even if we lazy load
* Add MemberListStore tests
* Additional tests
