Use Django's cache framework for service group names (#857)

Use django-redis-cache backend for locking get_or_set
No longer require group-related tasks to be locked to one simultaneous execution.
Remove legacy service group cache models.

Truncate Discord nicknames to 32 characters
Correct Discourse group name extension using only valid leading characters.
Prevent name slicing from ending with illegal character

Closes #801
Closes #847
Closes #835
Closes #852

alliance_auth/settings.py.example

@@ -211,10 +211,10 @@ MESSAGE_TAGS = {
 
 CACHES = {
     "default": {
-        "BACKEND": "django_redis.cache.RedisCache",
-        "LOCATION": "redis://127.0.0.1:6379/1",
+        "BACKEND": "redis_cache.RedisCache",
+        "LOCATION": "localhost:6379",
         "OPTIONS": {
-            "CLIENT_CLASS": "django_redis.client.DefaultClient",
+            "DB": 1,
         }
     }
 }