If a room was not joined (e.g., due to failure), `is_room` was wrongly returning `False`.
Disco identity is now used as fallback to detect correctly is an entity is a room or not.
This changes makes `is_room` async, and by extension `get_bare_or_full` too.
"Message Reactions" implementation is now working and updating history. The
`message_reactions_set` bridge method can be used by frontend to `replace`, `add`,
`remove` or `toggle` reactions. History is properly updated, and signal are sent when
necessary.
