WIP: Fetch YT / Vimeo preview images
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.
TODO:
- 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)