CLC-3811 CLC-3160 Simplify caching behavior & keys

https://jira.ets.berkeley.edu/jira/browse/CLC-3811
We've been treating view-as cache differently from real-user cache. Since caching is so core to CalCentral, this has thrown off QA & smoke testing, sometimes in very tense circumstances.

https://jira.ets.berkeley.edu/jira/browse/CLC-3160
Our earliest API feeds were all user-centric & so we naively included the string "user" in our default cache keys. Functionally speaking, all that's needed is an "ID", but the names in the code obscured that.

Since Cacheable is a mix-in & "key" is such a common method/variable name, it would probably be safest to collapse the three methods into "cache_key".