Show more detailed hints about draft revisions in the UI

Summary: Ref T2543. When revisions are in the draft state, tell the user what we're waiting for or why they aren't moving forward.

Test Plan: {F5228840}

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T2543

Differential Revision: https://secure.phabricator.com/D18714