R1:e516358d54d2

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…