Add a flag column to Differential revision lists

Summary:
Put a flag icon to the left of each flagged revision in the
Differential summary tables. Flag is colored correctly and when hovered
reveals the note in a tooltip.

As epriestley specifically notes that the Flagged Revisions table should
only be shown when a user is looking at their own revisions, maybe this
ought to be limited by what controller is using this view, or something.

Test Plan:
View Differential main page. Check that flags appear
correctly if some revisions are flagged.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin, avivey

Maniphest Tasks: T1557

Differential Revision: https://secure.phabricator.com/D3125