Fixed #19546 - ensure that deprecation warnings are shown during tests

refs #18985
Remove inaccurate comment regarding language names.
Skipped deprecation warning test on Python 2.6

Refs #19546. On Python 2.6, DeprecationWarnings are visible by
default.
Fixed lockups in jenkins, refs #19546.
Fixed #19586 - Removed URL_VALIDATOR_USER_AGENT from setting docs.

It was removed in Django 1.5, not deprecated.
Addeded CSS to bold deprecation notices.

Thanks Sam Lai for mentioning this on the mailing list.
Fixed #19581 -- ensure unique html ids with CheckboxSelectMultiple widgets

ID check is now done the same way as MultipleHiddenInput.
Fixed #19573 -- Allow override of username field label in AuthenticationForm
Fixed #19588 - Added create_superuser to UserManager docs.

Thanks minddust for the report.
Fixed #10239 - Added docs for modelform_factory

Thanks ingenieroariel for the suggestion and slurms for the review.
Fixed #19437 - Clarified pip install instructions in contributing tutorial.
Adapted test assertion against yaml dump

Fixes #12914 (again).
Fixed #19360 -- Raised an explicit exception for aggregates on date/time fields in sqlite3

Thanks lsaffre for the report and Chris Medrela for the initial
patch.
Made dev server autoreloader ignore filenames reported as None.

Useful under Jython. Thanks Thomas Bartelmess for the report and patch.

Ref #9589.
Fixed code examples in which render() calls were missing `request` parameter.
Minor DEBUG setting reference formatting edit.
Fixed #4833 -- Validate email addresses with localhost as domain
Fixed #19585 -- Fixed loading cookie value as a dict

This regression was introduced by the 'unicode_literals' patch.
Fixed #19488 -- Made i18n_patterns redirect work with non-slash-ending paths

Thanks Daniel Gerzo for the report and the initial patch.
Removed some now dead code from deletion (thanks to Carl Meyer for noticing it).