mix:referenceable is not set on namespace hgge, preventing making new newsletter documents in CMS

Newsletter documents can't be created in the CMS.

/hippo:namespaces/hgge must have the mix:referenceable, just like the other namespaces. The query for new-newsletter will otherwise not find any prototypes.

According to the bootstrap content (hgge.xml), the mixin is set; but this doesn't end up in the node.

To reproduce:

• Log in as editor
• Go to the documents, to the folder /hippogogreen/Newsletters
• Create a new newsletter
  Expected: I can enter a name and URL for the document
  Actual: A dialog with the message:
  "
  There are no types available for : [new-newsletter] First create document types please.
  "