Add course-unawareness option to ExperimentWaffleFlag

For the Courseware MFE rollout experiment, we want users'
default buckets to be consistent across course runs.

ExperimentWaffleFlag advised that this could be done
by calling `.is_enabled(...)` without a course_key argument;
however, doing so breaks when uing the main_flag.BUCKET_NUM
scheme to apply bucket rules for a specific set of users
or courses.

This commit explicitly adds course-unaware-bucketing via
a new kwarg to ExperimentWaffleFlag.__init__ method.

Furthermore, it fixes ExperimentWaffleFlag.is_enabled(course_key=None)
to work as advertised, by means of calling
.is_enabled_without_course_context on its subordinate flags.

TNL-7405

openedx/core/djangoapps/waffle_utils/__init__.py

@@ -477,7 +477,7 @@ class CourseWaffleFlag(WaffleFlag):
         """
         return self._is_enabled()
 
-    def is_enabled(self, course_key=None):
+    def is_enabled(self, course_key):
         """
         Returns whether or not the flag is enabled within the context of a given course.