Use `usegmt` flag in formatdate

Slightly cleaner and faster than string manipulation.
 
This flag has been available since Python 2.4:
http://docs.python.org/2/library/email.util.html#email.utils.formatdate
SQLite test fix -- refs #9057
Fixed #18356 -- Gave the test client signals.template_rendered call a unique dispatch_uid

This prevents the test client context from being lost when the client
is used in a nested fashion.
Merge pull request #1447 from evansd/patch-1

Use `usegmt` flag in formatdate
Fixed #20868 -- Added an email to django-announce as a security step.

Thanks garrison for the report.
Fixed #20828 -- Allowed @permission_required to take a list of permissions

Thanks Giggaflop for the suggestion.
Test that django.views.static.serve() generates 404 status codes.

Also, change tests to be based on SimpleTestCase.
Expand testing of Test LiveServerTestCase static files serving.
clarified misleading wording about squashing commits
Merge pull request #1464 from evildmp/squashing_commits

Clarified misleading wording about squashing commits
Fixed #20890 -- Added missing import in class-based view docs.

Thanks André Augusto.
Fixed #13518 -- Added FILE_UPLOAD_DIRECTORY_PERMISSIONS setting

This setting does for new directories what FILE_UPLOAD_PERMISSIONS
does for new files.

Thanks jacob@ for the suggestion.
Fixed #17778 -- Prevented class attributes on context from resolving as template variables.

Thanks KyleMac for the report, regebro for the patch, and Aymeric for the test.
Added missing release notes for older versions of Django
Unlocalize line numbers and ids in debug 500 view.

While using USE_L10N, line numbers and IDs were printed as comma (or
locale equivalent) separated values.

Thanks Kronuz for the report and intial patch.

Fixes #20861.
Fixed #20874 -- bump_prefix() in nested subqueries

Also made some cleanup to build_filter() code by introducing submethods
solve_lookup_type() and prepare_lookup_value().
Fixed overflow for the "Recent Actions" widget on the admin index.

Previously the CSS targeted "li.changelink" and therefore didn't
work for the "add" and "delete" actions.

Refs #14868.
Fixed #20883 -- Made model inheritance find parent links in abstract parents
Merge pull request #1470 from loic/admin.actionlist

Fixed overflow for the "Recent Actions" widget on the admin index.
Fixed test failure on MySQL

The fix for #20874 caused a MySQL specific failure.