Revert "Remove leave button from grouplist if current user is admin of that group."

People shouldn't be forced to be part of a group, even if they are the
admin. If a group has no admin, we need to figure out what to do with
it, but it's wrong to force anyone to be part of a group.

This reverts commit f9a7ae27b8.

lib/grouplist.php

@@ -164,10 +164,8 @@ class GroupList extends Widget
             # XXX: special-case for user looking at own
             # subscriptions page
             if ($user->isMember($this->group)) {
-                if (!$user->isAdmin($this->group)) {
                 $lf = new LeaveForm($this->out, $this->group);
                 $lf->show();
-                }
             } else {
                 $jf = new JoinForm($this->out, $this->group);
                 $jf->show();