Do not sync course registrations on login

[kylesferrazza]

This may be convenient, but became a performance issue.
Registrations can still be synced via the red Sync button for the course.

[blerner]

This is certainly the band-aid we need for now. But it looks like we can do better:

• maybe sync if hg_course.registrations.empty?
• we have a last_sync time -- could we get a timestamp from bottlenose of the most recent registration or withdrawal, and compare, and show a "Out of sync!" alert if bottlenose_time > last_sync?

[kylesferrazza]

• sync if hg_course.registrations.empty?
• we have a last_sync time -- could we get a timestamp from bottlenose of the most recent registration or withdrawal, and compare, and show a "Out of sync!" alert if bottlenose_time > last_sync?