Unify Differential/Maniphest/Diffusion styles and allow commits to be flagged explicitly

Summary:

• Differential, Maniphest and Diffusion use slightly different styles for the object detail panels.
• Instead, use the same styles and CSS.
• Add object actions to Diffusion, including "Flag".

Test Plan: Looked at revisions, tasks and commit. Flagged and unflagged commits.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran, epriestley

Maniphest Tasks: T1041

Differential Revision: https://secure.phabricator.com/D2062