Store region and carrier in a sensible way (bug 943539)

- Store SIM region and carrier in `region_sim` and `carrier_sim`
- Store GeoIP region in `region_geoip`
- Store override region and carrier in `region_override` and `carrier_override`
- Pull the region/carrier in this order:
  - URL
  - Override
  - SIM
  - GeoIP
  - (null; let the server decide)

- Fixes six broken tests
- Minor refactoring around URLs and buchets
- Removes redundant tests