Add u as a binding for universal-argument.
Change python-django-qmgmt-dbshell binding to "dsh".
Better management command output handling and callback triggering.

* python-django.el (python-django-mgmt-output-buffer): Remove var.
(python-django-mgmt-capture-output): Remove function.
(python-django-mgmt-run-command): Doc fix.
(python-django-qmgmt-define): Pass command to callbacks.
(python-django-qmgmt-kill-and-msg-callback): Use
display-message-or-buffer instead of message.
(python-django-qmgmt-dumpdata-callback): Use
buffer-substring-no-properties instead of
python-django-mgmt-output-buffer.