Add "Move Left" and "Move Right" to dashboard tab panels

Summary: Depends on D20474. Ref T13272. Provide an easy way to rearrange tabs on a tab panel, by moving them left or right from the context menu.

Test Plan: Moved tabs left and right. Tried to move them off the end of the tab list, no luck.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13272

Differential Revision: https://secure.phabricator.com/D20475