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

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.

This is duplicated, so let's factor it out before fixing it

this is internal to statestore, so let's keep it there.

Fix for #2635: correctly update rooms avatar/display name when modified by admin

This reverts commit f9b255cd, reversing
changes made to 1bd654da.

Return whether a user is an admin within a group

Ignore <noscript> tags when generating URL preview descriptions

Logging and logcontext fixes for Limiter

Add some logging to the Limiter in a similar spirit to the Linearizer, to help
debug issues.

Also fix a logcontext leak.

Also refactor slightly to avoid throwing exceptions.