Leaving a room shouldn't be blocking the whole interface

I'm currently trying to leave a bunch of old rooms, and for each of them Riot blocks the whole UI while waiting for the server to respond, which is pretty bad, especially when my HS becomes slow (in which case the app can stay blocked for quite some time).

Sorry if this is a duplicate, I tried to search for an existing issue about that but GitHub's issues search is pretty much crap.

Start by reproducing leaving multiple rooms while the homeserver is slow, then trace the room-leave request and the UI state that blocks interaction. Done means the rest of the interface remains usable while leaving is pending, with the leave operation still completing or reporting failure.

Written by the indexing model from the issue text.