Move actions into Diffusion header

Summary: This moves actions into the Diffusion main header, removes the locate file box, and widens description and cloning details. Projects are not currently in this layout, but will follow up in another diff. Trying to keep these changes small and iterative.

Test Plan:
Locate some files, test actions dropdown, repository with and without description. Also tablet, mobile layouts.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D18193