Add tabs to Diffusion for consistent navigation

Summary:
Adds a responsive tab bar navigation to Diffusion. Working through the new design here in pieces, so keep in mind M1477 is the target. Notably:

• Removes "branches" and "tags" from RevisionView, now on tabs
• Keeps "browse", "history", "readme" on RevisionView
• Adds tabs for all main views, including Graph... unless how that feels, so let me know.

Test Plan: Browse all pages, desktop and mobile. Test hg, svn, git repositories.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D18161