CLC-4181 API to let superusers delete specific cache entries

https://jira.ets.berkeley.edu/jira/browse/CLC-4181

This will assist our standard upgrade procedure and should also help when diagnosing or working around certain production issues.
No, @ctweney, the argument is not used at all in the Rails cache implementations or in Dalli. (It's hard to see how it would ever make sense functionally.)