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.
so https://bugzilla.mozilla.org/show_bug.cgi?id=1073177 looks fixed, but users still can't. is there a bug to actually fix this properly?