[lib] Remove community from thick threads
Summary:
Thick threads can't have a community.
https://linear.app/comm/issue/ENG-8937/remove-community-from-mutablethickrawthreadinfo
More context D12914.
Test Plan: Made sure that it's possible to create a sidebar. Also checked if persisting and reading thin and thick threads from SQLite works on web.
Reviewers: kamil, inka
Reviewed By: inka
Subscribers: ashoat
Differential Revision: https://phab.comm.dev/D12967