Documents often get shared over email, including access rights (read, write, comment) for the document.

I propose adding a new DigitalDocument (originally Document) type with the following subtypes:

• SpreadsheetDigitalDocument
• PresentationDigitalDocument
• TextDigitalDocument
• NoteDigitalDocument

To describe the permissions, a new DigitalDocumentPermission type would have the following properties:

• grantee
• permissionType

See pull request #1049 for en example.

Note: this description updated by @danbri to reflect naming changes agreed below (SpreadsheetDigitalDocument was SpreadsheetDocument, etc.).