Allow users to choose an active operator (bug 1074422)

Short video of the experience: ~~http://chuck.mn/2P2V452F3R1v~~ http://chuck.mn/311Q0u453Y2I

- 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