Use the standard enrollment table instead of a custom CCX table for CCX

enrollments.

The goal for this PR is to have a single mechanism for registering users and
reducing the number of places where special-casing for ccx courses is needed. The
migration at this point is purposefully limited to convert ccx memberships into
student enrollments when moving forward. No backward migration is in place at the
moment. The ccx membership tables are not removed at this time. It is possible to go
backwards and forwards multiple times with no errors or data loss.

openedx/core/djangoapps/content/course_overviews/models.py

@@ -15,6 +15,8 @@ from xmodule.error_module import ErrorDescriptor
 from xmodule.modulestore.django import modulestore
 from xmodule_django.models import CourseKeyField, UsageKeyField
 
+from ccx_keys.locator import CCXLocator
+
 
 class CourseOverview(TimeStampedModel):
     """
@@ -100,17 +102,26 @@ class CourseOverview(TimeStampedModel):
         except ValueError:
             lowest_passing_grade = None
 
+        display_name = course.display_name
+        start = course.start
+        end = course.end
+        if isinstance(course.id, CCXLocator):
+            from ccx.utils import get_ccx_from_ccx_locator  # pylint: disable=import-error
+            ccx = get_ccx_from_ccx_locator(course.id)
+            display_name = ccx.display_name
+            start = ccx.start
+            end = ccx.due
+
         return cls(
             version=cls.VERSION,
-
             id=course.id,
             _location=course.location,
-            display_name=course.display_name,
+            display_name=display_name,
             display_number_with_default=course.display_number_with_default,
             display_org_with_default=course.display_org_with_default,
 
-            start=course.start,
-            end=course.end,
+            start=start,
+            end=end,
             advertised_start=course.advertised_start,
 
             course_image_url=course_image_url(course),