Make __init__ consistent across Store hierarchy

Add a route for determining who you are

Signed-off-by: Travis Ralston <travpc@gmail.com>

Cache failures in url_preview handler

Fix 500 on invalid utf-8 in request

Fix the test to pass the right number of args to the Store constructors

Add db_conn parameters to the `__init__` methods of the *Store classes, so that
they are all consistent, which makes the multiple inheritance work correctly
(and so that we can later extract mixins which can be used in the slavedstores)

This reverts commit f5cf3638.

Useful for applications which may have an access token, but no idea as to who owns it.

Signed-off-by: Travis Ralston <travpc@gmail.com>

Reshuffle the caching logic in the url_preview handler so that failures are
cached (and to generally simplify things and fix the logcontext leaks).

Downcase userid on registration

Explain why we don't validate userids registered via app services

If somebody sends us a request where the the body is invalid utf-8, we should
return a 400 rather than a 500. (json.loads throws a UnicodeError in this
situation)

We might as well catch all Exceptions here: it seems very unlikely that we
would get a request that *isn't caused by invalid json.

Force username to lowercase before attempting to register

https://github.com/matrix-org/synapse/issues/2660

This reverts commit b70b6469.

Allow upper-case characters in mxids

Because we're never going to be able to fix this :'(

Namespace visibility options for groups

Fix 'NoneType' not iterable in /deactivate

Change so that update group room visibility isn't an upsert

make sure we actually return a value from user_delete_access_tokens

Fix appservices being backlogged and not receiving new events due to a bug in notify_interested_services

Avoid no-op media deletes

Fix bug in state group storage

Fix various embarrassing typos around user_directory and add some doc.

Fixes a bug where the persisted state groups were different to those actually
being used after auth resolution.