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.
Fixed #22887 -- Added comment for translators on unique_for_date error message
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
Fixed #22944: Bad dependency on FK alteration in autodetector
Fixed #23101 db_table wasn't copied in deconstruct

Forward port of 999758fc7a2d6fcb01eb40de937d4420f884788d from
stable/1.7.x
Fixed #23106 -- Updated class-based view reference title.
Fixed #22979 -- Moved bug* tests