Fetches preview images from Youtube or Vimeo if the ebucore:locator URI points to the respective video platform.
Since there are still a couple of questions open (e.g. how to check for existing thumbnail images on the sFTP server, see https://memobase.atlassian.net/browse/MEMO-712), the present merge request represents not a full implementation of all needed steps to enrich the record with thumbnail data.
Implement reporting for thumbnail enrichment
Real check for existing thumbnails on sFTP server (in which case the fetching of preview images on Youtube / Vimeo can be skipped)