Ability to invite a user to a room without them receiving an explicit invite.

Use case:
* I have a private room which I want any of a set of people to be able to join if they want.
* They know the alias.
* I want them to be able to (re)join and leave at will, without having a big red invite hanging around forever if they choose not to join it.

Start with the use case in the issue body and review the Matrix protocol specification's treatment of private rooms, aliases, invitations, joining, and leaving. Done means the specification clearly defines whether users can join or rejoin through the alias without receiving an explicit invite; no file or test path is provided.

Written by the indexing model from the issue text.