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
similar to [bug 1070084](https://bugzilla.mozilla.org/show_bug.cgi?id=1070084), which clouser unfortunately WONTFIX'd, this string arrangement is guaranteed to make grammatical sense only in English. I don't know how many non-English-speaking operators there are - but worth calling it.