feat: CourseEnrollmentAllowed API [BACKPORT][PALM] #526

Job	Run time
collect-and-verify (3.8, pinned)	2m 52s
gh-hosted-python-3.8,django-pinned,lms-1	11m 57s
gh-hosted-python-3.8,django-pinned,lms-2	8m 42s
gh-hosted-python-3.8,django-pinned,lms-3	16m 35s
gh-hosted-python-3.8,django-pinned,lms-4	20m 42s
gh-hosted-python-3.8,django-pinned,lms-5	9m 48s
gh-hosted-python-3.8,django-pinned,lms-6	15m 15s
gh-hosted-python-3.8,django-pinned,openedx-1	13m 34s
gh-hosted-python-3.8,django-pinned,openedx-2	14m 35s
gh-hosted-python-3.8,django-pinned,openedx-3	8m 6s
gh-hosted-python-3.8,django-pinned,openedx-4	4m 10s
gh-hosted-python-3.8,django-pinned,cms-1	3m 13s
gh-hosted-python-3.8,django-pinned,cms-2	15m 29s
gh-hosted-python-3.8,django-pinned,common-1	9m 56s
gh-hosted-python-3.8,django-pinned,common-2	5m 5s
gh-hosted-python-3.8,django-pinned,xmodule-1	11m 58s
	2h 51m 57s

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