Add minimal integration of UBA's openUMWELT repository using the existing OAI harvester.

We should probably still write a user story to look into improving metadata quality, possibly by using the product-specific metadataPrefix=dim (DSpace intermediate format) instead of the loosely standardized Dublin Core metadata, but for an initial pass this should suffice.

Sadly, it required loosening the XML schema almost everywhere as the systems dumps metadata into the DC prefix which cannot be distinguished there as the necessary attributes are not part of that schema. Hence, everywhere that happens, we just take the first value and hope for the best.