Fixed #20822 -- Set content type of default error pages to 'text/html'.

Thanks Jimmy Song for the patch.
Fixed #20858 -- Removed erroneous import in tutorial 2.

Thanks AtomicSpark.
Moved get_search_results from BaseModelAdmin to ModelAdmin.

Refs #15961.
Deprecated SortedDict (replaced with collections.OrderedDict)

Thanks Loic Bistuer for the review.
Fixed #20855 -- Added documentation of current_app and extra_context params to django.contrib.auth views

refs #5298 and refs #8342
Introduced ModelAdmin.get_fields() and refactored get_fieldsets() to use it.

Refs #18681.

This also starts the deprecation of ModelAdmin.declared_fieldsets
Fixed a number of lint warnings, particularly around unused variables.
Fixed #20842 and #20845 - Added a note on order_by() and improved prefetch_related() docs.
Fixed #20860 -- Removed references to defunct chicagocrime.org
Added some Selenium tests for the admin raw id widget.
Added a note to the 1.6 release about the new `--keep-pot` option for `makemessages`.
Refs #17008.
Fixed a small formatting issue.
Fixed some intermittent Selenium test failures.
Fixed #20862 -- Updated startproject MIDDLEWARE_CLASSES in docs.

Thanks Keryn Knight.
Fixed #20859 - Clarified Model.clean() example.
Fixed #20832 -- Enabled HTML password reset email

Added optional html_email_template_name parameter to password_reset view
and PasswordResetForm.
Fixed #20850 -- Added MultiWidget.needs_multipart_form
Fixed #17667 -- Prevented app loading from skipping nonexistent apps after the first try

Thanks ea2100@ for the report and akaariai for the patch.
Fixed #16986 -- Model.clean() can report errors on individual fields.

This commit provides the tests for this issue but the actual problem was solved
by the ValidationError refactor in f34cfec and ee77d4b.

Refs #20199.
Fixed ordering related test failure

Also PEP8 + python_2_unicode_compatible cleanup done.