Create document-derived classes to show different title labels.

Per [story 62665250](https://www.pivotaltracker.com/story/show/62665250). Interface for document/evidence uploading needs to have a heading corresponding to the type of evidence being uploaded.  This is looked up from the model's `title_singular` field.  Since only that needed to be changed, new classes are created that are the same as `CMS.Models.Document` except for that filed.