Add an explicit cutoff date for audit cert granting.

The previous logic was a convoluted way of doing the same thing, and
has already led to one bug. Instead of hoping that the bugs are ironed
out now and that future devs maintain this logic properly, let's just
set a real cutoff date.

lms/envs/common.py

@@ -2755,6 +2755,10 @@ MOBILE_APP_USER_AGENT_REGEXES = [
 DEPRECATED_ADVANCED_COMPONENT_TYPES = []
 
 
+# Cutoff date for granting audit certificates
+
+AUDIT_CERT_CUTOFF_DATE = None
+
 ################################ Settings for Credentials Service ################################
 
 CREDENTIALS_SERVICE_USERNAME = 'credentials_service_user'