[web] implement leaving DM thread

Summary:
ENG-8977

Updating component logic.

Depends on D13184

Test Plan:

1. Created thread with users A and B.
2. On A leave thread.
3. B see that A left, thread disappeared for A.

Reviewers: tomek, inka

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D13185