Fixed #17356 -- Allowed {% include %} to render compiled templates

Reviewed by Loic Bistuer and Tim Graham.
Fixed #7116 -- Optimize RequestContext construction
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
Removed stale add_q() comment
Fixed #20988 -- Added model meta option select_on_save

The option can be used to force pre 1.6 style SELECT on save behaviour.
This is needed in case the database returns zero updated rows even if
there is a matching row in the DB. One such case is PostgreSQL update
trigger that returns NULL.

Reviewed by Tim Graham.

Refs #16649
Improved {% include %} implementation

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.

Thanks loic84 for help with the tests.

Fixed #3544, fixed #12064, fixed #16147
Set 'bidi' Urdu property to True

Refs #20454.
Fixed #20989 -- Removed useless explicit list comprehensions.
Fixed #21005 -- Made schema tests language-independent

Thanks Simon Charette for the review.
Fixed #20999 - Allow overriding formfield class with choices, without subclass restrictions.

Refs #18162. Thanks claudep and mjtamlyn for review.
Fixed typo in docs/ref/models/options.txt
Fixed copy/paste error in measurement docs
Made the doc about translating string literals in templates more prominent.