Introduce new message types

Summary:
New message types: Sidebar Source and Create Sidebar
Things that need to be improved:

• Currently it's possible to create a sidebar only from text message. It was quite challenging to type it right, but I'll try to do that later.
• Message fetcher is unoptimal: we should fetch all the initial messages at once. I'm going to improve it in one of the next diffs.

Test Plan: Create two messages with new types in DB and check if they are displayed correctly

Reviewers: ashoat

Reviewed By: ashoat

Subscribers: KatPo, zrebcu411, Adrian, atul

Differential Revision: https://phabricator.ashoat.com/D573