Allow users to specify github user account / organization to publish repo in nextsteps.md

[rly]

A minor issue. The cookiecutter template asks for a github username and uses that to populate the

https://github.com/{{ github_username_list[0] }}/{{ cookiecutter.namespace }}/releases/tag/{{ cookiecutter.version }}
src: https://github.com/{{ github_username_list[0] }}/{{ cookiecutter.namespace }}/ndx-extracellular-channels

For the majority of cases, this is OK but users may want to use a github organization or different user account instead.