Introduce contributor role type instead of using text strings

Currently the contributor roles are just text strings which means that we have both Editor and Redakteur (German for editor) and we have Lektor (German for reader), but no equivalent English version.

We should introduce language independent objects for these which can then have multiple labels in different languages. One possible solution would be to use a curated list of Wikidata objects to be able to leverage their labels, descriptions, etc. Another possibility would be to look for a bibliographic source like MARC or Onyx.

This would require a database migration and would be a breaking change for any downstream consumers of the metadata (but the current metadata isn't really that useful anyway).

Related to #682, #2227, #2383