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
if you just always show a `<select>`, I think that'd be fine. fewer clicks when you have to change and simplifies the JS a bit.

![screenshot 2014-10-22 16 54 41](https://cloud.githubusercontent.com/assets/203725/4746262/dbfd6a8a-5a46-11e4-9063-ea5839968e7d.png)