DocumentTypesService: support fields of type Rich Text

Rich Text fields are similar to Formatter Text fields, but they also support links and embedded images.

Extend the Content Service module (back-end) to recognise and parse fields of type Rich Text. Rich Text is stored as a node.

On the REST API, both Rich Text fields and Formatted Text fields are mapped to a single field type "Rich Text", the difference between Rich Text and Formatted Text fields will be expressed through the field's CKEditor configuration.

CKEditor configuration support will be added as part of CHANNELMGR-1035.