Add 'governable' module to resources; DDR-1519.

For comparison ...

Old code: https://github.com/duke-libraries/ddr-models/blob/develop/lib/ddr/models/governable.rb

Old test: https://github.com/duke-libraries/ddr-models/blob/develop/spec/support/shared_examples_for_governables.rb

Dropped following methods from the code since I think they are no longer needed: #inherited_permissions, #inherited_rights, #copy_admin_policy_from, and the six #inherited_#{access}_#{type}s methods defined by meta-programming.