Add Content Type Gating Behind Waffle Flag

Content Type Gating: Xblocks that have a graded component cannot be
accessed by audit track users.
  - Caveats:
    - In studio, instructors can set certain xblocks to be available to
      all users, but graded components will default to not being
      available for audit users
    - If a course does not have a verified mode option, all users will
      have access to graded content.

The Waffle Flag: The waffle flag is of for now.
  It's name is: ```content_type_gating.debug```

This Commit Does NOT Include: Displaying for a user WHY they do not have
access to a specific piece of content.  That change will be part of
another PR.

common/lib/xmodule/xmodule/partitions/partitions.py

@@ -255,7 +255,7 @@ class UserPartition(namedtuple("UserPartition", "id name description groups sche
         """
         return None
 
-    def access_denied_fragment(self, block, user, course_key, user_group, allowed_groups):
+    def access_denied_fragment(self, block, user, user_group, allowed_groups):
         """
         Return an html fragment that should be displayed to the user when they are not allowed to access
         content managed by this partition, or None if there is no applicable message.

common/lib/xmodule/xmodule/partitions/partitions_service.py

@@ -8,10 +8,7 @@ from django.utils.translation import ugettext_lazy as _
 import logging
 
 from openedx.core.lib.cache_utils import request_cached
-from openedx.features.content_type_gating.partitions import (
-    CONTENT_GATING_PARTITION_ID,
-    create_content_gating_partition,
-)
+from openedx.features.content_type_gating.partitions import create_content_gating_partition
 from xmodule.partitions.partitions import (
     UserPartition,
     UserPartitionError,