Display a nice page when registration is not available

[MasloMaslane]

Display a nice page when registration is not available because of RegistrationAvailabilityConfig instead of 403

[MasloMaslane]

When RegistrationAvailabilityConfig is configured in contest settings so that the registration is not yet available

trying to register to a contest (/c/contest_id/register/) gives a 403 error. This is not informative enough, instead there should be a page that "Registration is not (yet) available"

[twalen]

I suggest to also fix small bug in the RegistrationAvailabilityConfig:

--- a/oioioi/contests/models.py
+++ b/oioioi/contests/models.py
@@ -410,7 +410,7 @@ class RegistrationAvailabilityConfig(models.Model):
             if self.registration_available_from is None or self.registration_available_to is None:
                 raise ValidationError(_("If registration availability is set to Configuration, then "
                                         "'Available from' and 'Available to' must be set."))
-            if self.available_from > self.registration_available_to:
+            if self.registration_available_from > self.registration_available_to:
                 raise ValidationError(_("'Available from' must be before 'available to'."))