[lib] Show parent thread members before community members

Summary:
https://linear.app/comm/issue/ENG-1710
When creating a subchannel, members of the parent chat should be listed at the top.

Test Plan: Start creating subchannels and check that there are no users with "not in parent chat" at the top of the list.

Reviewers: tomek, atul, abosh, ashoat

Reviewed By: tomek, ashoat

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D5159