Visibility status in show and list views

Creates a `Sufia::GenericFile::Visibility` module, used either in the model or its solr document, which define methods used to display information about a file's visibility.  Helper methods take care of displaying labels with the pertinent information.
How about splitting into `Sufia::GenericFile::Permissions::Readable` and `Sufia::GenericFile::Permissions::Writable`?  Then SolrDocument can include `Sufia::GenericFile::Permissions::Readable` and `Sufia::GenericFile::Permissions` can include both.