CLC-4052 Prevent duplicate calendar queue entries

https://jira.ets.berkeley.edu/jira/browse/CLC-4052

I also plan to add a unique index for class_calendar_queue on year/term/ccn/multi_entry_cd, and for class_calendar_log on year/term/ccn/multi_entry_cd/job_id. I'll do that next release, because the db migration filename change makes life difficult if migrations are present in the same release. I'll write a followup JIRA for the new indices .