Mention backward relationships in aggregate docs.

Thanks Anssi and Marc Tamlyn for reviewing.

Fixes #19803.
Fixed #19808 Capitalization error in example text
Merge pull request #719 from JonLoy/ticket_19808

Fixed #19808 -- Typo in example
Removed try-except in django.db.close_connection()

The reason was that the except clause needed to remove a connection
from the django.db.connections dict, but other parts of Django do not
expect this to happen. In addition the except clause was silently
swallowing the exception messages.

Refs #19707, special thanks to Carl Meyer for pointing out that this
approach should be taken.
Fix admindocs on Python 3, where None cannot be sorted with strings.

This fixes two tests in admin_views which were failing on Python 3, but only if
the tests were run with docutils installed.
Fixed #18558 -- Added url property to HttpResponseRedirect*

Thanks coolRR for the report.
Fixed #19693 -- Made truncatewords_html handle self-closing tags

Thanks sneawo for the report and Jonathan Loy for the patch.
Fixed #8404 -- Isolated auth password-related tests from custom templates
Fixed a misnamed variable introduced in commit 142ec8b283

Refs #8404.
Fixed #19819 - Improved template filter errors handling.

Wrap the Parser.compile_filter method call with a try/except and call the
newly added Parser.compile_filter_error(). Overwrite this method in the
DebugParser to throw the correct error.

Since this error was otherwise catched by the compile_function try/except
block the debugger highlighted the wrong line.
Fixed #19823 -- Fixed memcached code example in cache docs
Fix filtering during collection of paths in setup.py

Thanks Marek Brzóska for the report.
Fixed #19822 -- Added validation for uniqueness on USERNAME_FIELD on custom User models.

Thanks to Claude Peroz for the draft patch.