add /link-fedora-mobile endpoint for...linking fedora mobile. ;)

A lot of this code comes from the `/api/details` endpoint which is unfortunate because of code duplication, but I'm not sure generalizing it is worth the effort quite yet.

This is the endpoint that the Fedora Mobile app will hit once it has an API key. It allows it to register itself (by GCM registration ID). Once this happens, fmn.consumer will send it a verification message, and the user will have to hit accept. Then the device will ping the confirmation handler endpoint and complete the process.