Show the register button whenever FxA is enabled (bug 1081880)

This will change the `Sign In / Sign Up` buttons we have on the settings and purchases page to just be `Sign In` when using Persona. This is really gross, I am truly sorry.
Looks like these are defined in [`header.styl`](https://github.com/mozilla/fireplace/blob/master/src/media/css/header.styl#L252). I guess I'll move it there.