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.
this is all too specific. why do you need such specific selectors? if anything you should just need `.account-settings .extras`:

https://github.com/mozilla/fireplace/blob/master/src/media/css/account.styl#L105

but even that's a bit too specific.