Use the same image id for images with the same URL

[Rikuoja]

Currently, e.g. the Kulke importer creates separate image objects for each event even if the image URL is identical.

This results in spamming of identical images in the backend; also, as a side effect, the superevents will not have an image at all, as the importer does not recognize images with different ids to be identical even if the URL is the same.

Hence, consider making image URL field a unique field. Any duplicates (images with the same URL) should be combined into a single image object, with perhaps a n_events field telling the user how many events use this particular image.