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.
That is how it is supposed to work as far as I know. A logout button doesn't make sense because we can call the login code and get the logged in info again without any user input.