Added a comment for test of refs #20278.
Fixed #23883 -- Stopped flatatt modifying its argument
Corrected Permission.max_length in docs; refs #8162.
Added a test for model pre/post_init signals.
Fixed #23794 -- Fixed migrations crash when removing a field that's part of index/unique_together.
Fixed #23863 -- Made runtests accept the keepdb option.

refs #20550
Fixed typo in docs/internals/release-process.txt.
Fixed #23865 -- documented how to assign errors to a field in Model.clean()

Also added a unit test wit the simpler syntax which we have documented,
where the dictionary values are strings.
Fixed #21753 -- Raised exception when both `form_class` and `fields` are specified.
Fixed #23778 -- Added a doc section on using the Django runner for reusable apps.
Fixed typo in forms/models.py comment.
Fixed #23862 -- Made ManyToManyRel.get_related_field() respect to_field.
Fixed #23889 -- Added more usage of quote_name_unless_alias() to avoid deprecation warnings.
Updated an old note about GEOSGeometry.transform
Fixed #23887 -- Returned Bad Request for multipart parsing fails

Thanks Antti Häyrynen and Tim Graham for the report, and Aymeric
Augustin for the review.
Moved tests for render shortcuts to their own app.
Avoided rewrapping Contexts in render_to_response.

This change preserves backwards-compatibility for a very common misuse
of render_to_response which even occurred in the official documentation.

It fixes that misuse wherever it happened in the code base and docs.

Context.__init__ is documented as accepting a dict and nothing else.
Since Context is dict-like, Context(Context({})) could work to some
extent. However, things get complicated with RequestContext and that
gets in the way of refactoring the template engine. This is the real
rationale for this change.