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