Add a very basic cache status page

Summary:
Ref T5501. This is just getting version detection and availability right, probably.

Eventually, this will get lifted up a bit and "$remedy" will turn into setup issues (or maybe one setup issue saying "your cache setup is messed up, click here to understand why").

Test Plan:

I intend to shove these up to production one-by-one since production is APC and local is Opcache + APCu.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T5501

Differential Revision: https://secure.phabricator.com/D12314