Fxa email popup 1052876

https://bugzilla.mozilla.org/show_bug.cgi?id=1052876
https://bugzilla.mozilla.org/show_bug.cgi?id=1065725

When a "migratable" user starts the login they will be shown the FxA migration dialog describing the migration and redirecting them to login/register based on their email address.

![fxa-migration mov](https://cloud.githubusercontent.com/assets/211578/4391855/f04a9cf0-4408-11e4-9aa9-b9dde6fbb5ac.gif)

Where is this template used? Can't extend the base template (because you'll have to render via nunjucks, add the include files, cache busting stuff)?