Add member GitHub Actions follow-up work

[cunla]

The GitHub Workflow to add a new member works:
django-commons/membership#97

It can be more robust:

• Check that the user exists in GitHub
• Check that the user does not already exist in the list of members
• Check that there is no branch with the name add-user/...
• Have the action post a comment on the original issue with "please change the membership to public, ..."

[cunla]

• Check that the user exists in GitHub => DONE
• Check that the user does not already exist in the list of members => DONE