Separate commit message parsing and validation from Conduit

Summary:
Ref T11114. I want to move this step away from custom fields. To start with, isolate all the parsing in one class with a clearer API boundary.

Next, I'll make this class use new field objects to perform parsing, without CustomField interactions.

Test Plan: Created and edited revisions from the CLI, using valid and invalid commit messages.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11114

Differential Revision: https://secure.phabricator.com/D17055