Allow logged-in users to view and use the login form again

The fix for bug 15484 (d0439af8) has introduced the behavior of
automatically redirecting users who view the login form when already
logged in to the requested 'returnto' location. If it was not given,
users were redirected to main page instead.

However, that has annoyed people who often switch between several
accounts and have grown accustomed to the old behavior of being able
to log in while logged in. Given that there are no conflicts between
these two features, let's just restore the old behavior when
Special:UserLogin is visited directly (no 'returnto' location given).

This reverts 5dfc57eb which removed then-dead code for showing login
form to logged-in users and tweaks an if() condition.

Bug: 70855
Change-Id: I7e40c13a6ca566b4d66d943c006af9edb6941ee9
Merge "Allow logged-in users to view and use the login form again"