Make Differential objective markers show a brighter "editing" state

Summary:
Ref T12733.

• While editing a comment, show a pink star () with a tooltip.
• Slight UI tweaks, including draft comments getting an indigo pencil ().

Test Plan:

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12733

Differential Revision: https://secure.phabricator.com/D17977