Activate global completion for bash and zsh when package is installed

Presently this package doesn't actually activate completions unless a user knows to run activate-global-python-argcomplete once by hand. It would be best if this package also installed python-argcomplete into /etc/bash_completion.d as that script does (when run as root). Should do same for zsh. It seems that Debian does this, see https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=944469.

python-argcomplete is version 3.1.1-1.