Fixed #5849 -- Strip whitespace from blocktrans

Add the trimmed option to the blocktrans tag to trim any newlines and
whitespace from its content.

This allows the developer to indent the blocktrans tag without adding
new lines and whitespace to the msgid in the PO file.

Thanks to mpessas for the initial patch and Dmitri Fedortchenko for the
report.
Fixed a couple flake8 warnings.
Fixed #21396: Document backwards-incompatible change in RedirectView.get_redirect_url.

Thanks to Tim for the review.
Removed obsolete locale restriction admonition

Refs #14461. Thanks Ramiro Morales for pointing this.
Fixed #13970 -- Made SelectDateWidget use the standard widget is_required attribute

Thanks mitar for the report and Tim Graham for the review.
Fixed #17529 -- get_template_from_string default arguments break

``get_template_from_string`` default arguments were breaking
``assertTemplateUsed``. The solution has been to return only the names of the
templates with a ``name`` attribute distinct of ``None``. The default ``name``
kwarg of ``Template`` has been changed to ``None``, more pythonic than ``'<Unknown
Template>'``.
Fixed a typo in admindocs template

Thanks Reza Mohammadi for the report.
Flake8 fix -- correct number of newlines between top level definitions
Added missing parameter in npgettext_lazy docs
Fixed docs for EmailMessage extra_headers attribute

The EmailMessage documentation describes the initialization parameters
and states they 'can be set at any time prior to calling the send()
method.'. However, the 'headers' parameter's corresponding attribute is
called 'extra_headers'.
Noted that .clear() will delete intermediary models
Merge pull request #1743 from unaizalakain/ticket_21172

Fixed #21172 -- have LiveServerThread follow the semantics of threading.Thread.join()
Fixed #16969 -- Don't connect to named database when possible

Thanks Andreas Pelme for the report and initial patch, and
Aymeric Augustin, Shai Berger and Tim Graham for the reviews.
Removed useless comment. Refs #21172
Don't initialize PostGIS-specific stuff for non-db connections

Refs #16969.
Fixed #21383 -- Added request details in SuspiciousOperation messages
Recommended flake8 to check coding style.
Fixed #21378 -- Updated tutorial02 for how to override admin site header.
Fixed versionadded misuse in the admin documentation.
flake8 fix