Implemented #10164 for new schema migration code.

Made it use 'AUTOINCREMENT' suffix for PK creation. This way it doeesn't
regress when compared with the 'traditional' DB backend creation
infrastructure.

Refs #10164.
Worked around a bug in python 3.3.3. Refs #21093
Fixed #20346 -- Made cache middleware vary on the full URL.

Previously, only the URL path was included in the cache key.

Thanks jamey for the suggestion.
Fixed doc indentation in previous commit.
Introduced as_bytes for SafeMIMEText (and other SafeMIME-classes).

This is to provide a consistent interface (namely bytes) for the smtp
backend which after all sends bytes over the wire; encoding with as_string
yields different results since mails as unicode are not really specified.

as_string stays for backwardscompatibilty mostly and some debug outputs.
But keep in mind that the output doesn't match as_bytes!
Populated non-master app registries.

This removes the gap between the master app registry and ad-hoc app
registries created by the migration framework, specifically in terms
of behavior of the get_model[s] methods.

This commit contains a stealth feature that I'd rather not describe.
Stopped calling apps.get_model with only_installed=False.

ContentTypes are only created for installed applications, and I could
make a case for not returning a model that isn't installed any more.
The check for stale ContentTypes in update_contenttypes doesn't use
model_class.

ModelSignal actually needs get_registered_model since the lookup happens
at import time. I took this opportunity to perform a small refactoring.
Simplified Apps.get_model and added AppConfig.get_model.

Documented them as public APIs.
Changed get_model to raise an exception on errors.

Returning None on errors required unpythonic error checking and was
inconsistent with get_app_config.

get_model was a private API until the previous commit, but given that it
was certainly used in third party software, the change is explained in
the release notes.

Applied the same change to get_registered_model, which is a new private
API introduced during the recent refactoring.
Removed the only_installed argument of Apps.get_models.

Refs #15903, #15866, #15850.
Fixed #21242 -- Allowed more IANA schemes in URLValidator

Thanks Sascha Peilicke for the report and initial patch, and
Tim Graham for the review.
Added generation of SQLite FK DDL in initial migrations.

SQLite accepts the relevant standard SQL (although by default it doesn't
enforce the constraint), and the 'traditional' creation backend helper
generate it, so this allows us to:

- Maintain the status quo
- Improve readability of the SQL code generated for that backend.

Also, we will need this for when we fix Refs #14204.
Skipped inclusion of scripts (pip will take care of that).
Refactored the migration signals to use app configs.

De-aliased pre/post_syncdb to pre/post_migrate to increase
backwards-compatibility.
Cleared get_models cache when swapping User model.

Thanks Florian for isolating the shortest way to reproduce this issue:

./runtests.py \
    django.contrib.auth.tests.test_context_processors.AuthContextProcessorTests.test_perms_attrs \
    django.contrib.auth.tests.test_auth_backends.ChangedBackendSettingsTest.test_changed_backend_settings \
    django.contrib.auth.tests.test_auth_backends.CustomUserModelBackendAuthenticateTest.test_authenticate \
    django.contrib.auth.tests.test_basic.BasicTestCase.test_createsuperuser_management_command
Replaced Options.object_name.lower() by Options.model_name.

Thanks Simon for the suggestion.

Also removed inappropriate lowercasing of app labels in migrations.
Unlike model names, they are case sensitive.
Cleared global templatetags module cache.

TOOOOO MUCH GLOBAL STATE (you didn't see that).
Added AppConfig.get_models().
Fixed a typo.

Thanks Simon.
Added Apps.clear_cache().

This avoid leaking implementation details to tests that swap models.