Simplified the implementation of register_model.

register_model is called exactly once in the entire Django code base, at the
bottom of ModelBase.__new__:

    new_class._meta.apps.register_model(new_class._meta.app_label, new_class)

ModelBase.__new__ exits prematurely 120 lines earlier (sigh) if a model with
the same name is already registered:

    if new_class._meta.apps.get_registered_model(new_class._meta.app_label, name):
        return

(This isn't the exact code, but it's equivalent.)

apps.register_model and apps.get_registered_model are essentially a setter and
a getter for apps.all_models, and apps.register_model is the only setter. As a
consequence, new_class._meta.apps.all_models cannot change in-between.

Considering that name == new_class.__name__, we can conclude that
register_model(app_label, model) is always called with such arguments that
get_registered_model(app_label, model.__name__) returns None.

Considering that model._meta.model_name == model.__name__.lower(), and looking
at the implementation of register_model and get_registered_model, this proves
that self.all_models[app_label] doesn't contain model._meta.model_name in
register_model, allowing us to simplify the implementation.
Updated the AppCommand API to support apps without a models module.
Migrated built-in AppCommands to use handle_app_config.
Used app_label instead of appname.

The last component of the dotted path to the application module is
consistently referenced as the application "label". For instance it's
AppConfig.label. appname could be confused with AppConfig.name, which is
the full dotted path.
Fixed #21618 -- Added hints about openlayers.js hosting

Thanks kz26 for the report and Tim Graham for the excellent
review.
Fixed #21093 -- Ensured that mails are not base64 encoded on python 3.3.3+.

Thanks to Arfrever for the report and Aymeric for the review.
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.
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.