Ability to merge or redirect rooms

It's increasingly common (thanks to stuff like https://github.com/matrix-org/matrix-appservice-irc/issues/387) to end up with two Matrix rooms which serve the same logical purpose which you want to combine into one.

This is the spec version of the user-facing bug at https://github.com/vector-im/riot-web/issues/2299 to track this.

My proposal is for a `m.room.redirect` state event which Admins can set to mark a room as politely requesting that clients redirect everyone trying to join the room elsewhere.

Start with this proposal and read the linked matrix-appservice-irc and Riot issues to understand the room-merge problem. No files or tests are named, so first identify where room state events are specified. Done means reaching agreement on the redirect behavior and documenting the resulting protocol change.

Written by the indexing model from the issue text.