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