Fixed #19662 -- alter auth modelbackend to accept custom username fields

Thanks to Aymeric and Carl for the review.
Moved import in db.backends.creation
Fixed #19779 -- Checked contrib.sites presence in RedirectFallbackMiddleware

Thanks Aymeric Augustin for the report and directions for the patch.
Removed unneeded imports in db.models init
Cleaned up some lingering signals in the test suite that were causing spurious failures with Pypy and Postgres.
Fixed #18906 -- Ignored to-be-deleted forms in formset validate_unique

Thanks c.pollock at bangor.ac.uk for the report.
Fixed #19749 -- Documented ending param to command's self.stdout/err

Thanks xian at mintchaos.com for the report.
Fixed #19699 - Removed "Please see the release notes" from versionadded/changed directives

Also bumped django_next_version back to 1.6 so those
annotations are described as "the development version"
in the docs.

Thanks Aymeric for the patch.
Added tests for some sql_* management commands
Made modwsgi groups_for_user consistent with check_password

2b5f848207b1dab35afd6f63d0107629c76d4d9a based its changes on #19061
that made the is_active attribute mandatory for user models.
The try/except was not removed for the groups_for_user function.

refs #19780
Fixed #19707 -- Reset transaction state after requests
Fixed #19638 -- Skipped tx support testing for some DBs

PostgreSQL and Oracle always support transactions, so the create table
and rollback test is non-necessary on those DBs.

Thanks to shai for report.
Fixed #19720 -- Oracle ordering related delete regression

When a query had a complex where condition (a condition targeting more
than the base table) a subquery was used for deletion. However, the
query had default ordering from the model's meta and Oracle doesn't
work with ordered subqueries.

The regression was caused by fast-path deletion code introduced in
1cd6e04cd4f768bcd4385b75de433d497d938f82 for fixing #18676.

Thanks to Dylan Klomparens for the report.
Added a note about the default timezone and the new project template.

Thanks JonLoy for the draft patch.
Made Query.clear_ordering force_empty arg mandatory

Previously it was possible to call clear_ordering without the
force_empty argument. The result was that the query was still ordered
by model's meta ordering if that was defined. By making the arg
mandatory it will be easier to spot possible errors caused by assuming
clear_ordering will remove all ordering.

Thanks to Dylan Klomparens for the suggestion. Refs #19720.
Fixed #19112 -- Reduced the amount of query params in a test
Emphasized MyISAM pseudo-requirement for GeoDjango over MySQL

Refs #15295.
Updated tutorial 1 to reflect changes in default project template.

Thanks JonLoy for the patch.
Fixed #19044 -- Made `DeletionMixin` interpolate its `success_url`.

Thanks to nxvl and slurms for the initial patch, ptone for the review
and timo for the documentation tweaks.
Added a check in the creation of IS NULL clauses.

value_annotation isn't very well defined. Before this change, setting it
to datetime.datetime could silently reverse the behavior of isnull
lookups. This commit doesn't have any consequences on the current code.
It's just a safeguard for future ORM hackers.