Fixed #23900 -- Added missing imports in code example.

The Preventing header injection example included
classes that are not imported.

Thanks to Collin Anderson and Berker Peksağ for the
reviews.
Better migration docs for get_query_set -> get_queryset

This way they won't need to fix things again when get_query_set disappears
Removed unused variable django.http.request.absolute_http_url_re
Simplified get_version() in django/__init__.py

Thanks Luke Plant for the report.
Moved release note for refs #21281 from "deprecation" to "backwards incompatible".
Fixed spelling in docs/ref/django-admin.txt.
Added deprecation of template dirs parameter to timeline.
Fixed #23543 -- Added docs on testing management command output.
Further fixes to the migration notes for get_query_set

This rename is very tricky for the case of subclasses which define
get_query_set and haven't been updated yet, which applies to all projects in
the form of RelatedManager from Django 1.5.
Fixed typo in django/utils/decorators.py comment.
Updated testing documentation following 498ae3a36069e2d

- commit/rollback are no longer replaced by nop
- the warning about not using TestCase when testing transactional
  behavior belongs to TestCase section, not TransactionTestCase
Fixed #13694 -- Made SafeMIMEText's constructor compatible with MIMEText.
Removed workaround for lack of os.getpid() in Jython.

The Jython bug was fixed in http://bugs.jython.org/issue1518
(tested on Jython 2.7b3); also updated make_msgid() to be more like
the version in Python 3.2+; refs #23905.

Thanks Simon Charette for testing and review.
Fixed #23888 -- Fixed crash in File.__repr__() when name contains unicode.
Fixed #23907 -- Added admin.site.register in tutorial 2 for consistency.
Changed test_runner imports to name objects imported from test.runner.
Improved line breaking in runtests docs.
Fixed #23742 -- Added an option to reverse tests order.

This is useful for debugging side effects affecting tests that
are usually executed before a given test. Full suite and pair
tests sort cases more or less deterministically, thus some test
cross-dependencies are easier to reveal by reversing the order.

Thanks Preston Timmons for the review.
Fixed typo in docs/ref/django-admin.txt
Fixed #23890 -- Silenced numpy DeprecationWarnings in template tests.