Feature/delete by city

This replaces PR #173.  The `city_name` has been moved from `Indicator` to its related `IndicatorJob`.

In working on migrations for this, I noticed that #162 was missing a migration, and that the `unique_together` constraint on `Indicator` doesn't appear to actually be getting used in the database (likely a limitation of Django migrations.)
It's not about overhead, but debugging and being able to tell where something was imported from