Simplify SystemHookUrlValidator to inherit from PublicUrlValidator
Refactor specs to move out shared examples to be used in both
system hooks and public url validators.
Renamed UrlValidator to AddressableUrlValidator to avoid 'url:' naming collision with ActiveModel::Validations::UrlValidator in 'validates' statement.
Make use of the options attribute of the parent class ActiveModel::EachValidator.
Add more options: allow_nil, allow_blank, message.
Renamed 'protocols' option to 'schemes' to match the option naming from UrlValidator.