Make the Dashboard dropshadow a little lighter and turn panel management into a menu

Summary:
Depends on D20372. Ref T13272.

• There's a very heavy dropshadow on panels right now that looks out of place. Reduce it a bit.
• Panels currently have unlabeled pencil and trash icons. Turn this into a menu. I'm likely planning to add options like "Change Query..." to this menu to make managing some types of panels easier.

Test Plan: {F6332838}

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13272

Differential Revision: https://secure.phabricator.com/D20373