Add some tests for make_deferred_yieldable

Fix logcontext handling for persist_events

Add config option to auto-join new users to rooms

* don't use preserve_context_over_deferred, which is known broken.

* remove a redundant preserve_fn.

* add/improve some comments

The file under test is logcontext.py, not log_context.py

New users who register on the server will be dumped into all rooms in
auto_join_rooms in the config.

 Implement GET /groups/$groupId/invited_users

Log a warning when no profile for invited member

And return empty profile

Omit the *s for @room notifications

They're just redundant

 Basic implementation of backup media store