Fixed selenium failure.

This fixes (at least according to tests on the ci itself)
admin_widgets.tests.RelatedFieldWidgetSeleniumFirefoxTests.test_foreign_key_using_to_field

(http://ci.djangoproject.com/job/Django/database=mysql_gis,python=python2.7/3792/testReport/junit/admin_widgets.tests/RelatedFieldWidgetSeleniumFirefoxTests/test_foreign_key_using_to_field/)

The cause for this issue seems to be that wait_page_loaded was executed before click fired
and as such no profile got saved (again just an educated guess, but with this fix I can no
longer reproduce it -- fingers crossed).
Added build to flake8.exclude.
Suppressed ResourceWarning in files tests
Fixed #21728 -- Corrected an inadvertent path change in docs/intro/contributing.txt.
Fixed a sentence in the session security docs; thanks claudep.
Fixed #21730 -- Removed superfluous db_index in custom auth docs

Thanks rafalp for the report.
Fixed `python scripts/manage_translations.py update_catalogs`.

It used to generate languages 'e' and 'n'.
Removed useless model definitions.

Surprisingly, this commit doesn't change any behavior at all. When a
model is defined with the same name as another model in the same app,
the definition of the first class is bound to the name of the class
regardless of the definition of the second class.
Fixed test for models in non-installed apps.

Models are now attached to any application they're defined in. Since
not_installed was inside app_loading, these models were mistakenly
attached to app_loading. The test that used them passed accidentally
when run after EggLoadingTest because that class' tearDown() method
replaces apps.all_models['app_loading'] by a copy of itself, while
it should remain the same as apps.app_configs['app_loading'].models.
Fixed app registry cleanup in app-loading tests.
Renamed a couple models to prevent duplicate names.
Fixed #21711 -- Enforced unicity of model names.
Added an app-loading feature to the release notes.
Fixed #21675 -- Added app configs for contrib apps.
Added catalogs for apps that didn't have any translations.
Fixed #21718 -- Renamed has_app to is_installed.
Avoided catching ImproperlyConfigured on django.setup().

Fixed #21737. Thanks Florian for the report.

Also removed redundant imports in that file.
Changed parent class calls to super in Field subclasses
Fixed #21142: Dependency failures on unmigrated apps.
Fix wording of auth superuser post-migrate handler