R1:321b7761488d

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