Added HStoreField.

Thanks to `django-hstore` for inspiration in some areas, and many people
for reviews.
Added a warning about nonexistent FK constraints when unmigrated apps depend on migrated ones.

Thanks NotSqrt for the report; refs #23741.
Fixed links to mysqlclient in database notes.
Fixed #13181 -- Added support for callable choices to forms.ChoiceField

Thanks vanschelven and expleo for the initial patch.
Removed unused import.
Added missing docs to DiscoverRunner for keepdb option; refs #20550.
Fix minor typo in documentation.
Fixed #23531 -- Added CommonMiddleware.response_redirect_class.
Updated six to 1.8.0.
Removed confusing paragraph from the docs.

This snippet wasn't particularly helpful as `add_error()` is described
in greater details in the following section.

Thanks Claude Paroz for the report.
Added "Features removed in 1.8" section to release notes.
Removed redundant link in 1.7 release notes.
Added a dummy class for HTMLParserError; refs #23763.
Fixed #23770 -- Changed serialization strategy for floats with respect to NaN and Inf

Thanks to w0rp for the report
Fixed #23785 -- Typo in docs/releases/1.6.txt