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
b05948153a