crontabberise drop_old_partitions; bug 1014128; attempt #2

This addresses bug 1014128. Again. (Previous attempt at rebasing was a horrible disaster. :unamused:) 

`?r` @peterbe and/or @rhelmer :revolving_hearts: 
Fwiw, I used [`test_weekly_reports_partitions.py`](https://github.com/mozilla/socorro/blob/576c59b803560be75d8c376a091502de678e21d6/socorro/unittest/cron/jobs/test_weekly_reports_partitions.py) as the model, but the format changed since the initial PR. :smile: I'll review and adjust accordingly.