Support m.direct event in account_data

[giomfo]

see element-hq/element-web#2070:

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.