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
Probably a bad idea, now that I think about it. The current selection can (should?) be persisted so long as we make sure that when a new user logs in, that they have permission to work on that pair.

https://github.com/mozilla/marketplace-operator-dashboard/pull/1/files#diff-2f3941be5596154d17c7a0f95f2da3ccR79