[lib] Define permissions of PERSONAL and PRIVATE threads

Summary:
It shouldn't be possible to add a member or leave these threads.

https://linear.app/comm/issue/ENG-9382/update-personal-and-private-threads-permissions

Test Plan: Flow.

Reviewers: kamil, will, ashoat

Reviewed By: will, ashoat

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D13446