Implement updating DM thread subscription

Summary: This differential implements synchronizing thread subscription in dm threads.

Test Plan:

1. Create dm thread and add another user. (Use UI button implemented by Kamil)
2. Play around with changing thread subscription and sending notifs.

Reviewers: kamil, tomek

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D13236