Support for YubiKey

This PR adds support for two-factor authentication with YubiKeys. The PR is work-in-progress, and not yet ready for merge. It is listed as a PR to get code reviews / extra eyes early on.

* [x] Create unit test for setup view
* [x] Create unit test for login flow
* [x] Finalize setup instructions
* [x] Figure out how to `skipUnless` on Travis CI