CLC-3765 Add way to trigger HotPlate run

https://jira.ets.berkeley.edu/jira/browse/CLC-3765

Not a fix per se for the issue we saw, but a way to work around it. You can now hit /api/cache/hot_plate/{uid} and warm a single user. If {uid} is "all" it will warm up everyone. 

The clear_cache endpoint was changed to /api/cache/clear for clarity. I also added complete test coverage for the new CacheController.