Gitter badge link is incorrect

[kreczko]

Current link is

https://gitter.im/{{cookiecutter.url}}/community?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge

which leads to entries like

https://gitter.im/https://github.com/<account or org>/<project name>/community?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge

Instead, it should probably be {{cookiecutter.url}} → {{ cookiecutter.github_username }}/{{ cookiecutter.project_slug }}:

https://gitter.im/{{ cookiecutter.github_username }}/{{ cookiecutter.project_slug }}//community?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge

Does anyone know if cookiecutter.github_username also works for organizations? If yes, I can make a PR.