CLC-1971 Redo My Classes / My Groups around official student enrollments

https://jira.media.berkeley.edu/jira/browse/CLC-1971

- My Classes adds info for all enrolled sections, not just the first one (to support an upcoming My Academics UX change)
- My Classes is now restricted to sections in which I'm officially enrolled as a student, plus any LMS sites officially associated with those sections
- My Groups expands to include course sites that don't match the "My Classes" criteria
- My Activities now shows Canvas sources less generically

TO DO : Check performance! We now have at least 3 top-level REST API feeds with dependencies on the same hefty underlying data objects, & we don't want to refresh them multiple times simultaneously if we can help it.