upgrade to tutor.plugin.v1 and add parameter for custom theme repo url

[lpm0073]

This PR includes the following:

1. re-scaffolded the repository using cookiecutter-tutor-plugin in order to safely upgrade to tutor.plugin.v1
2. removed the explicit site_id parameter in manage.py create_or_update_site in templates/ecommerce/tasks/ecommerce/init
3. added a new parameter, ECOMMERCE_CUSTOM_THEME_REPOSITORY_URL, defaulted to ""
4. modified templates/ecommerce/build/ecommerce/Dockerfile to git clone the custom theme to the theme folder
5. added ENABLE_COMPREHENSIVE_THEMING to openedx-lms-production-settings patch to explicitly set to True if a URL is provided for the custom theme

[regisb]

Sorry about the delayed feedback @lpm0073. This PR was tackling too much stuff at once. If some features are still missing from the current master branch, please let's first discuss your changes in the forum.