Added an optional `backend` argument to login().

Thanks to bmispelon and uruz for the initial patch.

Thanks Jacek Bzdak for indepdently contributing a similar fix.

To improve consistency, sample URL names that had underscores
in them now use dashes instead. That excludes URL names that
have some relation to the code, such as those generated by
the admin.

Thanks guettli for reporting this.

Introduced an AbstractBaseSession model and hooks providing the option
of overriding the model class used by the session store and the session
store class used by the model.

When using a TransactionTestCase with serialized_rollback=True,
after creating the database and running its migrations (along with
emitting the post_migrate signal), the contents of the database
are serialized to _test_serialized_contents.

After the first test case, _fixture_teardown() would flush the
tables but then the post_migrate signal would be emitted and new
rows (with new PKs) would be created in the django_content_type
table. Then in any subsequent test cases in a suite,
_fixture_setup() attempts to deserialize the content of
 _test_serialized_contents, but these rows are identical to the
rows already in the database except for their PKs.  This causes an
IntegrityError due to the unique constraint in the
django_content_type table.

This change made it so that in the above scenario the post_migrate
signal is not emitted after flushing the tables, since it will be
repopulated during fixture_setup().

contrib.auth.mixins is based on our work with django-braces.

The new attribute is checked when the `migrate --fake-initial` option
is used. initial will be set to True for all initial migrations (this
is particularly useful when initial migrations are split) as well as
for squashed migrations.

When the Host header contains a port, looking up the Site record fails
as the host will never match the domain.

Added the mixins LoginRequiredMixin, PermissionRequiredMixin and
UserPassesTestMixin to contrib.auth as counterparts to the respective
view decorators.

The authentication mixins UserPassesTestMixin, LoginRequiredMixin and
PermissionRequiredMixin have been inspired by django-braces
<https://github.com/brack3t/django-braces/>

Thanks Raphael Michel for the initial patch, tests and docs on the PR
and Ana Balica, Kenneth Love, Marc Tamlyn, and Tim Graham for the
review.

Made some contributions in the past, seems like I never got around to adding myself :)

https://github.com/django/django/commits?author=alimony

Updated the command to match the documentation, which states it
runs over all .po files.