Added test for the #23070 problem
Fixed code to solve #23070 problem

Added a class to wrap callable in settings:
* Not to call in the debug page (#21345).
* Not to break the debug page if the callable forbidding to set attributes (#23070).

Thanks @bmispelon for giving me some advice.
Merge pull request #2967 from hirokiky/fix23070

Fixed #23070 -- not to break the debug page by callable setting with __slots__
Fixed #22305 -- Added note to docs about UTF8 database requirement.
Fixed #23103 -- Added contributing tutorial link to contributing index.
Fixed #16617 -- Added 'value' to BaseValidator params.

Also allowed overriding the default messages in subclasses of BaseValidator.

Thanks sperrygrove for initial patch.
Fixed #22553 -- Added refreshing queryset info to docs.
Fixed #22873 -- Renamed use_debug_cursor to force_debug_cursor to clarify the behavior.
Fixed #23060 -- Prevented UnicodeDecodeError in debug templatetag