Add endpoint for retrieving template attributions and more metadata

Adds endpoint /room/<roomId>/contentgroup/<id>/attributions to retrieve licenses and attributions for all contents of a group:

[{
  "contentId": "<UUID>",
  "license": "CC-BY-4.0",
  "attribution": "John Doe"
}]

Furthermore, this MR adds the attribution and aiGenerated attributes to content group templates.

Closes: #224 (closed)