Fixed #20881 -- Removed contrib.auth.models.AbstractUser.get_absolute_url()

The definition is arbitrary and creates a broken "view on site"
link in the admin if a project doesn't define such a URL.
Fixed #16433 -- Fixed a help_text/read only field interaction that caused an admin crash.

Thanks chris at cogdon.org for the report and admackin for the patch.
Fixed #14786 -- Fixed get_db_prep_lookup calling get_prep_value twice if prepared is False.

Thanks homm for the report and Aramgutang and lrekucki for work on
the patch.
Fixed #20989 -- Removed explicit list comprehension inside dict() and tuple()

Thanks jeroen.pulles at redslider.net for the suggestion and
helper script.
Added links to file docs.
Fixed #20997 -- Added SessionStore note to docs.

Thanks jsdalton.
Added SplitHiddenDateTimeWidget to django.forms.widgets.__all__
Altered test introduced in f19a3669b8 for the sake of readability. refs #14786
Fixed #3544, #12064, #16147 -- {% include %} behavior

Merged BaseIncludeNode, ConstantIncludeNode and Include node.

This avoids raising TemplateDoesNotExist at parsing time, allows recursion
when passing a literal template name, and should make TEMPLATE_DEBUG behavior
consistant.
Added test for #16147