Allow URL link to image in event creation API
This has been developed for https://keskonfai.fr/ in the API (but not available in the user interface). This avoids duplicating images and saves storage. A side effect it that if the event is viewed a lot, it will use the bandwith of the source website. Notes regarding this aspect: -a warning should be added in the API documentation, so that users of this feature are aware and make sure that the source website allows this use -on the long term we could consider using a local cache