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.
Merge pull request #1432 from loic/modeladmin.get_search_results

Moved get_search_results from BaseModelAdmin to ModelAdmin.
Fixed #20855 -- Added documentation of current_app and extra_context params to django.contrib.auth views

refs #5298 and refs #8342