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.
Would it make sense for parity's sake to also define the following convenience method?

```ruby
def private?
  !(public? || registered?)
end
```