[server] Use message specs in message fetchers
Test Plan: Logged out, then logged in and opened a thread with couple of different messages (text, images, sidebar source, some other robotext) and check if all of them were correct.
Reviewers: ashoat
Reviewed By: ashoat
Subscribers: KatPo, zrebcu411, Adrian, atul, subnub
Differential Revision: https://phabricator.ashoat.com/D632