BE side for pleroma-fe#201

Basically, the same API as /api/statuses/dm_timeline.json but showing one message per thread, the last one. Probably group messages by participants and sort groups by message latest message time.

The idea is to display threads of DMs grouped by participants, and be able to show more threads at same time without fetching more and more of them and having to do thread-filtering logic on FE.

Tell me if it's possible. If not - i'll figure out how to do it on FE, should be possible with just /api/statuses/dm_timeline.json, i think.