Fixed #21639 -- Implemented RenderContext.__getitem__

It's now consistent with RenderContext.get.
Small flake8 fixes -- number of blank lines between top level definitions
Set stacklevel for the override_settings warning.

Refs #19031.
Revert "Changed admin/templates/admin/index.html to make <caption> translatable"

This reverts commit a25fe3b65e39ff6f59ac545d7934461be6c13612.

It didn't do anything. It merely added to every PO file:

msgid "%(name)s"
msgstr "%(name)s"

Thanks Ramiro Morales for the report at:
https://groups.google.com/d/msg/Django-I18N/vc2vQzv80UQ/EuaW38V7X7sJ

Conflicts:
	django/contrib/admin/templates/admin/index.html