Add "diffusion.createcomment" conduit endpoint

Summary:
Adds most of Diffusion's commenting options available in the web UI

Mark method as deprecated immediately per @epriestley's request

Test Plan:
Used the Conduit web console to check:
* Lookup by PHID works
* Error is raised if commit by PHID is not found
* "action" validation works and raises appropriate error
* "message" raises error if empty
* Actions to raise concern or accept commit work
* Method is marked as deprecated from the start

Reviewers: epriestley

Reviewed By: epriestley

CC: Korvin, aran

Differential Revision: https://secure.phabricator.com/D6923