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:

pythondotorg/events/utils.py

Lines 23 to 26 in 8a02c10

	def extract_date_or_datetime(dt):
	if isinstance(dt, datetime.datetime):
	return convert_dt_to_aware(dt)
	return dt

Since the ICS importer is often met with a string that looks like a date-only, like this one:

pythondotorg/events/tests/test_importer.py

Line 39 in 8a02c10

DTSTART;VALUE=DATE:20160402

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:

pythondotorg/events/importer.py

Lines 33 to 37 in 74e659c

	# Let's mark those occurrences as 'all-day'.
	all_day = (
	dt_start.resolution == DATE_RESOLUTION or
	dt_end.resolution == DATE_RESOLUTION
	)

which has now been updated.