Ability to notify moderators in a room

It would be useful for `@mods` or similar to bing all users in a room with PL > default, as a blunt way of summoning moderators in face of abuse (even if there isn't a moderator space, or aristotle-style rules set up)

No file, test, or entry point is named. Start by locating the Matrix specification sections defining room notifications and power levels, then establish the required moderator-notification semantics and acceptance behavior before proposing a change.

Written by the indexing model from the issue text.