allow email domain to be defined in config/config.yml

* cross-fork config skeletons to config/cross-fork-dev/
* added cross-fork merged Gemfile to config/cross-fork-dev/
* added cross-fork-dev-README.md to config/cross-fork-dev/
* removed the hard-coded default mail domain in config/environment/production.rb that was explicitly over-riding the one defined in config/config.yml