Fixed #23690 - fixed examples of manual rendering of form fields

Documentation for rendering form fields manually is now updated to use fields id_for_label instead of hardcoded values with additional mention of label_tag for alternative generation of complete label tag.
Fixed #23585 - Corrected internal comment.

Removed misleading comment and provide correct one, explaining
idea behind hardcoded CSRF template context processor.
Fixes a race condition in the documentation.

The example for django.contrib.admin.ModelAdmin.get_form modifies
self.exclude. However, since ModelAdmin instances are global and have no
thread- or request-locality, this is not safe for concurrent
requests.[1] This updated documentation demonstrates a safe method to
override admin forms on a per-request basis.

[1] https://groups.google.com/forum/#!topic/django-users/AmoUDtEefyA
Designated Django 1.8 as the next LTS.
Merge pull request #3524 from nmundar/ticket_23690

Fixed #23690 - fixed examples of manual rendering of form fields
Merge pull request #3526 from gregorth/ticket_23585

Fixed #23585 - Corrected internal comment.
Merge pull request #3549 from psagers/master

Fixes a race condition in the documentation.

The example for django.contrib.admin.ModelAdmin.get_form was modifying self.exclude. However, since ModelAdmin instances are global and have no thread- or request-locality, this is not safe for concurrent requests. This updated documentation demonstrates a safe method to override admin forms on a per-request basis.
Renamed qn to compiler
Added reminder to committing docs to test a PR with Jenkins first.
Fixed #21612 -- Made QuerySet.update() respect to_field
Fixed regression in 4dc4d12e. Refs #21598.

That commit contained a mistake that resulted in the use_cached_loader
option of override_with_test_loader being ignored. As a consequence some
configurations weren't exercised any more by the test suite.
Moved all template loaders under django.template.loaders.

Reformatted the code of base.Loader according to modern standards.

Turned the test template loader into a regular locmem.Loader -- but
didn't document it.

Added a normal deprecation path for BaseLoader which is a public API.

Added an accelerated deprecation path for TestTemplateLoader which is
a private API.
Removed the "test:" prefix from locmem template identifiers.

Since it isn't branded as a test utility any more and could be used for
other purposes than test code, that prefix no longer makes sense.

It wasn't used anywhere either.
Removed skip_template argument of locmem.Loader.load_template_source.

It didn't do anything, wasn't documented and wasn't used anywhere.
Refactored listing template subdirectories in apps.

This change has the nice side effect of removing code that ran at import
time and depended on the app registry at module level -- a notorious
cause of AppRegistryNotReady exceptions.
Removed override_template_loaders and override_with_test_loader.

They can be replaced with override_settings and that makes the
corresponding tests much more obvious.
Removed obsolete comment.

It didn't account for class-based template loaders.
Deprecated function-based loaders.
Refactored getting the list of template loaders.

This provides the opportunity to move utility functions specific to the
Django Template Language outside of django.template.loader.
Used get_template_loaders in the cached loader.

This ensures that enabling the cached loader doesn't change behavior.

(Before this commit, it did when the list contained unusable loaders.)