Validate commit message fields on the server side

Summary:
See T643. We have some hard-coded checks in Arcanist for the existence of
'testPlan' and 'title', and don't properly validate those fields on the server.
Add a validation pass in the Conduit-based edit pathway.

In particular, this means that if you disable the "Test Plan" field, Arcanist
won't block you anymore.

Test Plan: Disabled Arcanist checks and ran "arc diff"; got blocked on the
server side.

Reviewers: btrahan, jungejason

Reviewed By: btrahan

CC: aran, btrahan

Differential Revision: 1153