Fixed ticket #21172

LiveServerThread.join() now behaves like threading.Thread.join().
LiveServerThread.terminate() is instead used to ask live http server to
terminate and close.
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
Fixed doc typo.
Fixed #21398 -- Fixed BCryptSHA256PasswordHasher with py-bcrypt and Python 3.

Thanks arjan at anymore.nl for the report.
Fixed spelling of compatibility.
Fixed #14800 -- Suppressed WSGIRequestHandler message filtering

Filtering out static file requests in runserver has been judged
arbitrary and can hide some debugging-related activity.
Thanks Roy Smith for the report and Aymeric Augustin for the
review.
Fixed #21372 -- Corrected docs regarding translating LANGUAGES.

Corrected LANGUAGES documentation on how to translate language
names. Now using django.utils.translation.ugettext_lazy instead
of a dummy gettext() function.

Thanks to Salvatore for the report.
Fixed typo in tutorial 2; refs #21418.
added description of MySQL Connector/Python

Reorganized the MySQLdb section into a "MySQL DB API Drivers" section that describes both MySQLdb and MySQL Connector/Python. Included description of the Django adaptors for these DB API drivers.