R1:ae0348aac931

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

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