[lib] Extract user ids from DM ops

Summary: issue: ENG-8829

Test Plan: Tested by dispatching a create_thread op, and then an add_members to the just created thick thread. Checked that the user id was added to redux.

Reviewers: tomek, kamil

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D13205