Show README on repository screen in Diffusion
Summary:
- Show README on the repository screen.
- Move README to the bottom of the page for both repository and browse screens.
- Support "README.rainbow".
Test Plan: Looked at repository, browse screens. Made a "README.rainbow".
Reviewers: btrahan, vrana, jungejason
Reviewed By: btrahan
CC: aran
Maniphest Tasks: T1104
Differential Revision: https://secure.phabricator.com/D2336