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 guess this tool probably won't get a lot of requests, but it's generally a good idea to read from an in-memory `object` when possible (and when it's defined, populate it with values in `localStorage`).