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
I can't wait until we use real Promises (and use [this polyfill](https://github.com/jakearchibald/es6-promise)).