Some improvements

[blag]

• [x] Better distance calculation when import districts
• [x] Move add_continents to cities.util
• [x] Add DistrictsTestCaseMixin (TestCase)
• [x] Move test mixins to their own module (test_project.mixins)
• [x] Add tests for custom continent data
• [x] Renamed ambiguous CITIES_IGNORE_EMPTY_REGIONS option to CITIES_SKIP_CITIES_WITH_EMPTY_REGIONS
• [x] Add districts to test data