CLC-2178 Eliminate duplicate Final-Grades activity entries

JIRA: https://jira.media.berkeley.edu/jira/browse/CLC-2178

As far as we can tell, the duplicates occurred because the is_dupe? check used event.timestamp, while the notifications themselves got created with occurred_at = event.eventNotification.event.timestamp, which can be a different value. I tweaked the test suite to check this situation, and wrote a migrator that removes duplicate notifications. 

When this pull is accepted I'll make another for QA, and then a third to update master's previous_release_db_schema.txt accordingly (since this is a post-branch patch).