hide kerberos tab on login page if we did not log out recently (fixes #5407)

Kerberos login should be automatic most of the time.

src/authentic2_auth_kerberos/auth_frontends.py

@@ -21,3 +21,6 @@ class KerberosFrontend(object):
 
     def template(self):
         return 'authentic2_auth_kerberos/login.html'
+
+    def is_hidden(self, request):
+        return 'a2_just_logged_out' not in request.COOKIES