Add separate visibility to problems

Tests are missing, but existing ones were adapted to the new
functionality.

Author: schrjako

web/problems/models.py

@@ -6,6 +6,7 @@
 from django.db import models
 from django.template.defaultfilters import slugify
 from django.template.loader import render_to_string
+from django.utils.translation import gettext_lazy as _
 from django.urls import reverse
 from django.utils.translation import gettext_lazy as _
 from rest_framework.authtoken.models import Token