refactor: update `all_day` detection logic

[miketheman]

Description

Instead of using a particular resolution on an object, which differs between datetime.date and datetime.datetime objects, operate on parent class of datetime and convert all dt to timezone-aware datetime values.

This is also in accordance to the model field being a DateTimeField, so we should always be passing the correctly-created object, instead of a datetime.date(), raising received a naive datetime warnings.

Removes unused constants.

Closes

Closes #2415

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Other notes, for fun.

The warnings stem from this little bitty function:

https://github.com/python/pythondotorg/blob/8a02c109590e6e81aa05f8213e7fde7a3d05858f/events/utils.py#L23-L26

Since the ICS importer is often met with a string that looks like a date-only, like this one: https://github.com/python/pythondotorg/blob/8a02c109590e6e81aa05f8213e7fde7a3d05858f/events/tests/test_importer.py#L39

the returned type is datetime.date, not a datetime.datetime.

The change in events/utils.py drops the warnings, but fails two test cases:

======================================================================
FAIL: test_import_event_excludes_ending_day_when_all_day_is_true (events.tests.test_importer.EventsImporterTestCase.test_import_event_excludes_ending_day_when_all_day_is_true)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/code/events/tests/test_importer.py", line 131, in test_import_event_excludes_ending_day_when_all_day_is_true
    self.assertTrue(all_day_event.next_or_previous_time.all_day)
AssertionError: False is not true

======================================================================
FAIL: test_modified_event (events.tests.test_importer.EventsImporterTestCase.test_modified_event)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/code/events/tests/test_importer.py", line 63, in test_modified_event
    self.assertTrue(e.next_or_previous_time.all_day)
AssertionError: False is not true

These come from the all_day calculation over here:

https://github.com/python/pythondotorg/blob/74e659c33077839657bd702a9eb1ddd678ebc5c4/events/importer.py#L33-L37

which has now been updated.