www/rubygem-railties70: Add rubygem-railties70 7.0.0

Railties is responsible for gluing all frameworks together. Overall, it:

• handles the bootstrapping process for a Rails application;
• manages the rails command line interface;
• and provides the Rails generators core.

WWW: https://rubyonrails.org/
WWW: https://github.com/rails/rails/tree/master/railties