These are surprisingly expensive, and we only really need to do them at startup.

Fixes #6815

Before figuring out whether we should alert a user on MAU, we call get_notice_room_for_user to get some info on the existing server notices room for this user. This function, if the room doesn't exist, creates it and invites the user in it. This means that, if we decide later that no server notice is needed, the user gets invited in a room with no message in it. This happens at every restart of the server, since the room ID returned by get_notice_room_for_user is cached.

This PR fixes that by moving the inviting bit to a dedicated function, that's only called when the server actually needs to send a notice to the user. A potential issue with this approach is that the room that's created by get_notice_room_for_user doesn't match how that same function looks for an existing room (i.e. it creates a room that doesn't have an invite or a join for the current user in it, so it could lead to a new room being created each time a user syncs), but I'm not sure this is a problem given it's cached until the server restarts, so that function won't run very often.

It also renames get_notice_room_for_user into get_or_create_notice_room_for_user to make what it does clearer.

this is never set to anything other than "test", and is a source of unnecessary
boilerplate.

The expected use case is to suppress MAU limiting on small instances

The `expire_access_token` didn't do what it sounded like it should do. What it
actually did was make Synapse enforce the 'time' caveat on macaroons used as
access tokens, but since our access token macaroons never contained such a
caveat, it was always a no-op.

(The code to add 'time' caveats was removed back in v0.18.5, in #1656)

Migrate all tests to use the dict-based config format instead of hanging items off HomeserverConfig (#5171)

Make sure that we have a `server_notices_mxid` set, given that we are relying
on it.

This test stubbed out some stuff in a very weird way. I have no idea why. It broke.

This manifested in synapse repeatedly setting the tag for the room

This was due to a bug where we mutated a cached event's contents