Backend for tracking 1:1 room state

[ara4n]

We've finally defined the behaviour for tracking 1:1 rooms:

• the idea is to use User account_data to track the list of DMs we see for a given remote user
• which would have a m.direct event in it, whose content gives a bunch of mxid to list-of-roomid mappings (we could split these into separate m.direct events, but these feel ugly)
• for bootstrapping: the first time Vector loads without any m.direct event, i'd expect it to synthesize one from the current heuristics of what counts as a 1:1 room.

An open problem is how to pick which 1:1 to use when selecting a user in #1882. From MemberInfo we can list all the rooms you have in common with the user and let the user sort it out. But from the Start Conversation component it's less clear. Will raise that there.