Instead of relying on the local avatar/displayname of a user, request the data from the server and update the pill if it shows up.
This required a slight refactor which means we're not doing everything in `render` now. Also I noticed unknown rooms weren't being rendered _at all_! So now you get something that looks like a normal link but with the room alias/ID in it.
This isn't an entirely side-effect-free refactoring:
- the text of the timeline pills is now either the room ID/alias or user ID/ display name of the linked resource (which means that until we do a roundtrip to get user displaynames, mentions for users not in the current room will have their user IDs shown instead of what was in the link body).
- timeline links to rooms without avatars are now rendered as links
- fixed issue that would throw an error whilst rendering (i.e. unusable client) a room link to a room that the client doesn't know about
