DocumentTypesService: support fields of type Formatted Text

Extend the Content Service module (back-end) to recognise and parse fields of type Formatted Text. As FormattedText is stored as an attribute of type String, it seems likely that we'll have to extend the StringFieldType class in order to support Formatted Text.

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.