[server] Introduce message specs and use them in message creator

Test Plan: I sent a text and image message and left a thread. Then logged in as another user and checked if all three messages were present.

Reviewers: ashoat

Reviewed By: ashoat

Subscribers: KatPo, zrebcu411, Adrian, atul, subnub

Differential Revision: https://phabricator.ashoat.com/D629