Webclient room data restructure

This works for both live and paginated events. Each 'message' event has
an associated '__room_member' key which points to the state of the sender
at that point in time. Invites have an additional key '__target_room_member'
which point to the state of the invitee at that point in time. This obviates
the need for mapping user_ids to *current* displaynames in the message list,
though this is still required for the user/presence list.

Reference the room_member key on messages which adjusts for current vs old_room_state. This displays names for historical users correctly, but is off by one (referencing content not prev_content).

Everything now uses modelService, but there are still one or two
minor teething problems to fix.

This breaks functionality whilst both events.rooms and modelService
are in use.

Prevent EventStreamService from knowing too much about the EventHandlerService by changing the contract to just be a single initialSync response callback. Leave it up the handler to deal with splitting out information from /initialSync.

room.html now displays messages from model-service. Add debugging fields. Hook up the room member *at the time* to the message so it can display the right historical member info.