Creating and deleting aliases could be more consistent

This issue has been migrated from [#7320](https://github.com/matrix-org/synapse/issues/7320).

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This is the promised follow-up to #7052 about how to creating and deleting aliases could be more consistent. In general these are implemented via the `create_association` and `delete_association` methods of `DirectoryHandler`.

The checks done for each of these (copied from #7052 and clarified a bit) below. They're a bit confusing (and maybe make more sense as the flow-charts I wrote out), but the overall checks aren't too complicated.

### Creation

1. Check if the length is valid.
1. If `require_membership_for_aliases` is true in the config, the creator must be in the room or be a server admin.
2. The spam checker's `user_may_create_room_alias` method is queried.
3. The `alias_creation_rules` are checked from the config.
4. Services are checked to see whether they have an "exclusive lock" on an alias.

### Deletion

1. Ensure that the user is the room creator, if one exists.
2. Ensure that the alias points to a room ID.
3. The user must be one of the following:
1. A server admin.
2. In the room and have sufficient power level to delete an alias (>= moderator).
5. Services are checked to see whether the have an "exclusive lock" on an alias.

## Potential improvements

There aren't too many changes I came up with, but see ideas below:

1. Check `require_room_membership_for_aliases` during alias deletion.
2. Ensure that a user has the proper power level during creation (but that probably only makes sense for the canonical alias, which already happens).

There's a chance that some of the code could be re-used by combining some of the checks, but probably not. Frankly this might just be good enough the way it is.