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.
why don't you just do this:

```css
.persona.register {
  display: none;
}
.only-logged-out .persona.register {
  display: inline-block
}
```