[bug 896484] Track the number of active contributors per locale/product.

* Added a new utility function to get # of active contributors
  per locale+product
* Added a new monthly cron job that calls it for each locale+product
  and saves the results to the WikiMetric table
* Created a new migration that backfills the data for this year
* Tests for the utility and cron job

Also, fixed some buggy tests that broke when I added my new tests.

r?