Add Reporter.is_active flag

This resolves issue #21.

* Added is_active flag to chws.Reporter model
* Added management command to calculate whether Reporters are active or not
* Added custom sql to add the column
* Updated view to utilize the is_active flag for CHWS Active/Inactive reports
* Further customized admin views to facilitate lookups.

The above changes drop the database queries from @25000 to @120; the reports render w/in 1.5 seconds vs 10 minutes (or crashing due to running out of memory).