Fixed #21511 -- Adding django_bash_completion to the setup.py script packaging of Django.

Submitting a pull request for this ticket: https://code.djangoproject.com/ticket/21511

Description:
I can't seem to get to django_bash_completion from a pypi package install of Django. I believe this is due to not including the package for "extras". Additionally, if django_bash_completion was added as an explicit script, it would be placed in the $VIRTUAL_ENV/bin/ folder for easier use.