What's missing? Being able to use separate rooms & lists for typing
indicators.
At the moment, we use the same ones as we use for the timeline, as
todo_rooms is quite intertwined. We need to disentangle this to get that
functionality, although I'm not sure if clients use it.
- Remove most usages of `update_membership` in favor
of directly calling the `mark_as_*` functions
- Store the leave membership event as the value in the
`userroomid_leftstate` table
- Use the `userroomid_leftstate` table to synchronize the
timeline and state for left rooms if possible
