Allow to override COURSE_ENROLLMENT_MODES in AWS environment

Add more parameters to COURSE_ENROLLMENT_MODES to allow to display friendly mode name and more options

Fix python tests of PR #18557

PR #18557: Fix Diff Quality test line too long

PR #18557: Fix Diff Quality test unused-variable

Use six.iteritems() in COURSE_MODE_SLUG_CHOICES for future Python 3 compatibility

cms/djangoapps/contentstore/views/tests/test_item.py

@@ -384,7 +384,7 @@ class GetItemTest(ItemTest):
                 "scheme": "enrollment_track",
                 "groups": [
                     {
-                        "id": settings.COURSE_ENROLLMENT_MODES["audit"],
+                        "id": settings.COURSE_ENROLLMENT_MODES["audit"]["id"],
                         "name": "Audit",
                         "selected": False,
                         "deleted": False,