Fix regress added to migrations.test_operations.test_alter_field_pk_fk

Fixed regression introduced by fabc678f. Thanks Michael Manfre
for the report.

Fixed #21751 -- Explicitly closed cursors.

This commit touchs various parts of the code base and test framework. Any
found usage of opening a cursor for the sake of initializing a connection
has been replaced with 'ensure_connection()'.

Updated SQLUpdateCompiler.execute_sql to match the behavior described in
the docstring; the 'first non-empty query' will now include all queries,
not just the main and first related update.

Added CURSOR and NO_RESULTS result_type constants to make the usages more
self documenting and allow execute_sql to explicitly close the cursor when
it is no longer needed.

Thanks Russell for the advice.

Fixed #21923. Refs #21719.

The purpose of this construct is to test if the django.contrib.sites
application is installed. But in Django 1.9 it will be forbidden to
import the Site model when the django.contrib.sites application isn't
installed.

No model besides Site used this pattern.

Refs #21719, #21923.

Thanks Timo for the report. Regression from 2f65b8e1. Refs #21794.

Thanks dfunckt for the report.

Broke InspectDBTestCase.test_field_types in two:
- a test_number_field_types, which now passes on Oracle too
- a test_field_types, for all non-numeric fields, which is still expected to fail

Also made some pep8 fixes in the tests file. Refs #19884

Thanks Tim Graham for review.

Fix typo CRSF -> CSRF

Fixed failing test schema.tests.SchemaTests.test_add_field_temp_default_boolean
Refs #19884

It seems this case was fixed somewhere between 1.5.x and 1.6.x. I added
tests as I wasn't able to find any tests for these cases. Refs #21879

When moving code around from sql.where to lookups six.moves.xrange
import was forgotten. The xrange import is needed on Python 3.

The combination of BaseManager.from_queryset() and RenameMethodsBase results in
Manager.__module__ having the wrong value. This can be an issue when trying to
pickle the Manager class.

Wherever possible this filesystem path is derived automatically from the app
module's ``__path__`` and ``__file__`` attributes (this avoids any
backwards-compatibility problems).

AppConfig allows specifying an app's filesystem location explicitly, which
overrides all autodetection based on ``__path__`` and ``__file__``. This
permits Django to support any type of module as an app (namespace packages,
fake modules, modules loaded by other hypothetical non-filesystem module
loaders), as long as the app is configured with an explicit filesystem path.

Thanks Aymeric for review and discussion.

Fixed #21688. Refs https://github.com/django/django/pull/1147.

The translation.override context manager cleans up after itself. As a
consequence this mixin isn't needed any more in many cases.

It contained models, forms and admin objects causing undesirable
import side effects. Refs #16368.

Thanks to Ramiro, Carl and Loïc for the review.

Thanks Wim for the report.