Implement joining DM thread
Summary: This differential refactors useJoinThread hook so that it handles DM threads as well.
Test Plan: TODO. Once Inka implements creating thick sidebars testing will be easy and smooth.
Reviewers: kamil, tomek, inka, ashoat
Reviewed By: ashoat
Subscribers: ashoat
Differential Revision: https://phab.comm.dev/D13260