Add icons to the new repository edit nav

Summary:
Ref T10748. These:

• Look nice.
• Hint at panel contents / effects.
• Hint which panels have been customized.
• Allow panels with issues or errors to be highlighted with an alert/attention icon.

Test Plan: {F1256156}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10748

Differential Revision: https://secure.phabricator.com/D15836