[lib] Extract the unsupported sidebar source message types to one single source of truth

Summary:
As mentioned in the previous diff, we want a single source of truth for a list of message types that should disallow sidebar creation. This diff moves the check into a helper function in
message-utils.js, and adds a quick unit test alongside this to check the integrity of the method.

Depends on D9237

Addresses part of ENG-4849

Test Plan: Ran flow, confirmed that the test plan in D9237 still works, and ran yarn workspace lib test to see the unit tests passing

Reviewers: ashoat, atul, ginsu

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D9238