Remove django-shorturls with my own implementation, since it's incompatible with Django 2 and unmaintained

lemonshort/short_url.py

@@ -0,0 +1,16 @@
+from django.apps import apps
+from django.conf import settings
+
+from .convert import id_to_abc
+
+prefixes = {}
+
+
+def short_url(entity):
+    if not prefixes:
+        for k, m in settings.SHORTEN_MODELS.items():
+            prefixes[apps.get_model(m)] = k
+    base = '/'
+    if hasattr(settings, 'SHORT_BASE_URL'):
+        base = settings.SHORT_BASE_URL
+    return base + prefixes[type(entity)] + id_to_abc(entity.id)