Modernize Diffusion "change" view

Summary:
  - Kicks it out to full width.
  - More useful header/crumbs/properties/actions (needs some more work).
  - Works for public repositories.
  - Fix a bug where the "rX" crumb would lose the branch you're on.

Test Plan: See screenshot.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D7063