Eliminating the create_mods_stream method from NuCollections.  Shouldn't be needed any longer.
Updated factories file to spin up a basic, valid Collections object
Adds tests for custom setters/getters on the NuCollection object.  Also added a describe block describing the janky behavior of parent set and minimal testing for embargo date setting.
NuCollection now tests the case where no embargo date has been set.
Removed a single bunk mods datastream test.  Updated mass_permissions= to clear perms so that registered and public cannot both exist in the rights metadata stream.  Setting to private clears both.
Adds the ability to set mass read permissions for the public/registered (or neither) groups for a collection during creation.  Dropdown also prefills with the correct value when inside the edit action.