Remove URI from Reference
leave it only for ref type WebSite and maybe Database and Generic(?).
General links to representations of a reference should be stored as "ExternalLink"s. But these are currently only available for OriginalSource, not directly for IdentifiableEntity, needs to be discussed.
copy from #7918: "currently available for most types but will be removed in future except for websites (and maybe databases). Links to online available references should not be stored in URI but in the new "ExternalLink" which will be related as a collection of external links available for this publication. This is because there might be multiple links available for the same reference and it also allows a description what exactly stands the link for. Currently this is already available for the OriginalSource class to link to sources (e.g. single pages in a reference)."