Fixed #21177 -- Made resolve_url support relative URLs.

This fixes redirecting to relative URLs with django.shortcuts.redirect.
Fixed #17942 -- Added a JsonResponse class to more easily create JSON encoded responses.

Thanks leahculver for the suggestion and Erik Romijn,
Simon Charette, and Marc Tamlyn for the reviews.
Fixed #21924 -- Added the ability to specify a reverse order for admin_order_field.

Thanks Klemens Mantzos for the report and initial patch.
Fixed #22035 -- reordered migration operations

Now AddField actions appear in operations list before AlterUniqueTogether
actions.

Thanks to SmileyChris for the report.
Added 1.6.3 release note for refs #21566.