Add dialog to purge opcode/data caches

Summary: Reachable via the cache config page, restricted to admins only. This makes it convenient to hotfix phabricator without requiring a restart.

Test Plan:

• Local dev machine doesn't have apc, so I get the not installed message.
• Faked the name and isEnabled parameters, verified dialog shows up as expected.
• Didn't test clear code

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: tycho.tatitscheff, joshuaspence, Korvin

Differential Revision: https://secure.phabricator.com/D14064