Merge TSParticipants in TSThread

This pull request makes small changes to TSThread in order to get rid of the TSParticipants class. It also simplifies TSThread's API so that there's only one constructor instead of 2 (that were actually doing very similar things). I made the assumption that the current user is always a participant of the threads they can access on their device.
I also re-added the checks in TSHKDF for null salts that were removed.

My long term goal (if I have time to work on this) is to simplify how messages and threads (including axolotl variables) work, and how they're stored in the DB. I'll push smaller increments/changes than what you've seen before :p. I understand that you guys are also working on this so feel free to reject this request if it's not what you had in mind.