Fixed #22903 -- Fixed migration generation if index_together or unique_together is removed from a model.

Thanks Walter Doekes.

Previously, GET, POST, and FILES on an HttpRequest were created in
the __init__ method as dictionaries. This was not something you would
usually notice causing trouble in production as you'd only see a
WSGIRequest, but in testing using the test client, calling .getlist
on GET, POST, or FILES for a request with no get/post data resulted in
an AttributeError.

Changed GET and POST on an HttpRequest object to be mutable
QueryDicts (mutable because the Django tests, and probably many
third party tests, were expecting it).

Now QueryDict() is equivalent to QueryDict('') or QueryDict(None).

Function definition should have an ending colon.

Thanks bmispelon, kezabelle, jorgecarleitao, and timgraham for reviews.

Thanks GDorn and gox21 for report.

Thanks Tim Graham for idea and review.

Thanks Florian Apolloner for raising the issue.

Thanks Víðir Valberg Guðmundsson for the report and
Tim Graham for the review.

Thanks to @SmileyChris for the report and @timgraham for review.

This enables using a custom user model with a primary key with a name
different than 'id'; refs #22889.

Refs #17215. In the same spirit as 5de31cb8.

Refs #17215.

Refs #21482.

Thanks Tim Graham and Aymeric Augustin for the review.

This follows commits 80f4487d and 01399fa0; original patch had to be
reverted because it wasn't Python 2.6 compatible and we need it to
be in order to build docs on the djangoproject.com server.

This fix should be replaced by @lru_cache as soon as we drop
Python 2.6 compatibility.

Thanks Florian Apolloner for the review and Alexander Schepanovski
for the original patch.

The old location was removed in cebd8753.
Thanks Aymeric Augustin for the report.

As far as possible, message evaluation should not happen in field
__init__ (often run at import time).

Undocumented, untested and probably not even useful feature.