Fixed #23893: Added tzinfo to constant datetime in unit test

Added tzinfo to y2k constant (01/01/2000) in
custom_lookups.tests.DateTimeLookupTests.test_datetime_output_field
to fix warning message regarding naive datetimes.
Merge pull request #3737 from mhall1/ticket_23893

Fixed #23893: Added tzinfo to constant datetime in unit test
Fixed #23995 -- Removed unnecessary calls to self.client.logout() in tests.
Fixed #23983 -- Fixed a crash in migrations when adding order_with_respect_to to non-empty table.
Fixed #16028 -- Moved defaultfilters tests into template_tests.
Fixed #23975 -- Restored pre_migrate signal if all apps have migrations.

Thanks kmmbvnr for the report.
Fixed the formatting of one section of the security page