CLC-2608 Fix Canvas site cache handling

https://jira.media.berkeley.edu/jira/browse/CLC-2608

This PR clarifies that CanvasUserSites is a merged cached feed but not an API model.

Before I went on vacation, @ctweney & I agreed on the desirability of breaking up the current spaghetti tangle of the merged Canvas & Sakai feeds, probably by moving MyClasses and MyGroups to a merged module-based approach. I hope that will land before (or as part of) CLC-2631.