fix thumbnailing (#2548)

`os.path.exists` doesn't allow us to distinguish between permissions errors and
the path actually not existing, which repeatedly confuses people. It also means
that we try to overwrite existing key files, which is super-confusing. (cf
issues #2455, #2379). Use os.stat instead.

Also, don't recomemnd the the use of --generate-config, which screws everything
up if you're using debian (cf #2455).

Remove dead class

Add some tests for make_deferred_yieldable

Fix logcontext handling for persist_events

Add config option to auto-join new users to rooms

in commit 0e28281a the code for thumbnailing got refactored and the
renaming of this variables was not done correctly.

Signed-Off-by: Matthias Kesler <krombel@krombel.de>

* 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

This isn't used anywhere.

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