Add __toString to Content document

Background

In different cases we have to display the name/title of a document. Currently not all documents support this.

Functional requirements

• Add a __toString function to the content document
• Add the __toString function to all document types (you can refer to the Solr configuration for the "title" field to see the data it need to contain
• You will need to make the Content document abstract, because it can't have a __toString
• An implementation has already been created, but was denied: INTEGRATED-364 Closed