Split message types definitions into multiple files
Summary: First part of the refactoring: all the message types have their own file. The only exception is SIDEBAR_SOURCE which has to be in the same file as union types definition due to potential circular dependency.
Test Plan: Flow, smoke test
Reviewers: ashoat
Reviewed By: ashoat
Subscribers: KatPo, zrebcu411, Adrian, atul, subnub
Differential Revision: https://phabricator.ashoat.com/D603