Add the default enrollment start date on course creation (#30954)

* 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

xmodule/tests/test_course_block.py

@@ -4,20 +4,22 @@
 import itertools
 import unittest
 from datetime import datetime, timedelta
+import sys
 from unittest.mock import Mock, patch
 
-import pytest
 import ddt
 from dateutil import parser
 from django.conf import settings
 from django.test import override_settings
 from fs.memoryfs import MemoryFS
 from opaque_keys.edx.keys import CourseKey
+import pytest
 from pytz import utc
 from xblock.runtime import DictKeyValueStore, KvsFieldData
 
 from openedx.core.lib.teams_config import TeamsConfig, DEFAULT_COURSE_RUN_MAX_TEAM_SIZE
 import xmodule.course_block
+from xmodule.course_metadata_utils import DEFAULT_START_DATE
 from xmodule.data import CertificatesDisplayBehaviors
 from xmodule.modulestore.xml import ImportSystem, XMLModuleStore
 from xmodule.modulestore.exceptions import InvalidProctoringProvider
@@ -32,10 +34,22 @@ _LAST_WEEK = _TODAY - timedelta(days=7)
 _NEXT_WEEK = _TODAY + timedelta(days=7)
 
 
-class CourseFieldsTestCase(unittest.TestCase):
+@ddt.ddt()
+class CourseFieldsTestCase(unittest.TestCase):  # lint-amnesty, pylint: disable=missing-class-docstring
 
     def test_default_start_date(self):
-        assert xmodule.course_block.CourseFields.start.default == datetime(2030, 1, 1, tzinfo=utc)
+        assert xmodule.course_block.CourseFields.start.default == DEFAULT_START_DATE
+
+    @ddt.data(True, False)
+    def test_default_enrollment_start_date(self, should_have_default_enroll_start):
+        features = settings.FEATURES.copy()
+        features['CREATE_COURSE_WITH_DEFAULT_ENROLLMENT_START_DATE'] = should_have_default_enroll_start
+        with override_settings(FEATURES=features):
+            # reimport, so settings override could take effect
+            del sys.modules['xmodule.course_block']
+            import xmodule.course_block  # lint-amnesty, pylint: disable=redefined-outer-name, reimported
+            expected = DEFAULT_START_DATE if should_have_default_enroll_start else None
+            assert xmodule.course_block.CourseFields.enrollment_start.default == expected
 
 
 class DummySystem(ImportSystem):  # lint-amnesty, pylint: disable=abstract-method, missing-class-docstring