Notifications based on space membership

A really useful thing would be able to bing users who are members of a given space - similar to `@room`, but for all users who are a member of a given set of users (i.e. are members of a staff space, or moderator space, or whatever)

Start by reviewing how the Matrix specification defines @room notifications, spaces, and membership, then identify the protocol entry points affected by notifying members of a selected space. Done means the behavior and membership rules are specified clearly enough for clients and servers to implement consistently.

Written by the indexing model from the issue text.