Cache README content for repositories

Summary:
Ref T11954. Especially with higher-latency file stores like S3, we can spend a lot of time reading README data and then pulling it out of file storage.

Instead, cache it.

Test Plan: Browsed a repostory with a README, saw faster pages.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11954

Differential Revision: https://secure.phabricator.com/D17002