Allow users to choose an active operator (bug 1074422)

Short video of the experience: http://chuck.mn/2P2V452F3R1v

- The list of carrier/region pairs is pulled from the API on login.
- Visuals of the experience are pending suggestions from @pwalm
- I'll make a few line comments to explain some things.

r? @ngokevin @spasovski @cvan
unless you're referring to something else, just set `offline_cache_enabled` to `false` in your `settings_local.js`. (FYI, from Fireplace's Debug screen, there's a button to disable/clear offline cache.)