Wireframe the UX for room version upgrades

[lampholder]

• Ability for an op to view the current room version and upgrade it by setting an m.room.tombstone (in settings) - by hitting some new CS API for /upgrade_room, perhaps (which gives us the option to do auto_join stuff automagically serverside in future)
• Display m.room.tombstones as a sticky message at the bottom of the old room (perhaps replacing the message composer input) as “This room has been upgraded, please click here to continue” or similar.
• On clicking on an m.room.tombstone, the client switches to the new room_id pointed to by the tombstone.
• If you try to join a room that your server is too old to support, you should see a sensible error message
• It should try to join - if the join fails (due to server being too old) we show the “Sorry, your server is too old to participate in this room” error.
• If the client is correctly joined to the new room, it should hide the old one from the RoomList.
• If one backpaginates the new room to its creation, we should show the “m.room.create” as “This room is a continuation of a previous room; click here to view” (or similar).
• Search is extended to check the full set of room_ids for this room (could be done clientside for now).

[lampholder]

First draft mockup with a variety of ideas for room versioning UX:

https://www.figma.com/file/Gz4d0BZQBWchNFu7UgeZx8S8/Room-Versioning?node-id=0%3A1

All feedback super welcome