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
since you're checking `operators` [above](#diff-2f3941be5596154d17c7a0f95f2da3ccR73), maybe you could just add an `if (operators)` around both of these `if`s.