[lib] Support other initial message types

Summary: All the types but SIDEBAR_SOURCE are supported

Test Plan: Created a new SIDEBAR SOURCE message in db, pointing to an image message. Verified that it was displayed correctly.

Reviewers: ashoat

Reviewed By: ashoat

Subscribers: KatPo, zrebcu411, Adrian, atul

Differential Revision: https://phabricator.ashoat.com/D577