Reduce drag-and-drop jank on dashboards

Summary:
Depends on D20414. Ref T13272. Several minor things here:

• Currently, you can drag panels underneath the invisible "there are no items in this column" div and the "Create Panel / Add Existing Panel" buttons. This is silly; stop it.
• Currently, when viewing a tab panel on a dashboard, you can drag the panels inside it. This is extremely silly. Make "movable" off by default and pass it through the async flow only when we actually need it.
• Make the whole "Add Tab..." virtual tab clickable to open the dropdown. This removes the rare exception/todo combo I added earlier. F
• Add or remove some icons or something.

Test Plan: Moved panels around on dashboards. Tried to drag panels inside tab panels. Added tab. Things were less obviously broken.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13272

Differential Revision: https://secure.phabricator.com/D20415