[lib][native][web] Introduce threadTypeIsSidebar

Summary: In this diff we replace threadTypes.SIDEBAR checks with threadTypeIsSidebar, a new utility function that also checks threadTypes.THICK_SIDEBAR. This allows our business logic to treat the two in the same way.

Test Plan: I followed the test plan described in this Linear comment, specifically for sidebar creation

Reviewers: tomek, kamil, marcin

Reviewed By: tomek

Differential Revision: https://phab.comm.dev/D12839