Bug 1099009 - Implement login 3.0 for non-Angular pages

At insignificant performance cost, hiding all and showing the right one is a more readable approach:

```Javascript
adminEl.addClass("hidden");
supermentorEl.addClass("hidden");
mentorEl.addClass("hidden");

if (user.isAdmin) {
  adminEl.removeClass("hidden");
}
else if (user.isSuperMentor) {
  supermentorEl.removeClass("hidden");
}
else if (user.isMentor) {
  mentorEl.removeClass("hidden");
}
```