* fix: main page course listing
The course is visible on the main page right after creation when the feature toggle `CREATE_COURSE_WITH_DEFAULT_ENROLLMENT_START_DATE` is on.
So anonymous users can see them and access the course about page
for the courses without valid data (e.g. they will see the default
course overview)
When courses list filtering is processed it checks the `see_exists`
permission for the anonymous user.
Actually, `see_exists` means `can_load` OR `can_enroll`.
`can_load` is False in our case because the course start in the future.
But `can_enroll` returns True because the course's enrollment_start
and enrollment_end dates are blank:
```
enrollment_start = courselike.enrollment_start or datetime.min.replace(tzinfo=UTC)
enrollment_end = courselike.enrollment_end or datetime.max.replace(tzinfo=UTC)
if enrollment_start < now < enrollment_end:
debug("Allow: in enrollment period")
return ACCESS_GRANTED
```
Set the enrollment_start the same as a course start by default
