Add support for machine readable metadata licenses

[asmecher]

Currently, published content can be licensed in a machine-readable way using settings in the journal setup (or equivalent for OMP and OPS). These settings are stamped on individual submissions, accommodating possible change of journal license without altering the license for already-published content.

However, this license is generally understood to apply to the published content (the article full-text). The submission metadata may be licensed differently -- often more permissively, for example, to facilitate indexing.

Consider adding support for a journal-level metadata field for metadata licensing.

Considerations:

• We probably don't want to stamp the license on submissions like we do with the existing license field. Just about all uses of metadata will need to apply in aggregate (e.g. listing records via the OAI-PMH interface) and unless we're able to state what the metadata license is for the entire journal, chaos will ensue.
• It's not clear what the downstream support for metadata licenses is -- JATS, CrossRef, etc. Unless we can find indications that there is meaningful support for this in other systems, we may want to hold off (unless we consider it valuable enough that just presenting it to readers somewhere in the About pages is enough).

[asmecher]

Thanks to @rcgillis for the heads-up -- OAI-PMH does have tools for describing metadata licenses, both at repository (Identify verb) and set level: https://www.openarchives.org/OAI/2.0/guidelines-rights.htm#repository_level

[AhemNason]

I'm just noting here for the record (because we chatted externally) that Crossref treats all incoming metadata (because of their API and agreement with members) as being "without copyright". https://www.crossref.org/documentation/retrieve-metadata/rest-api/rest-api-metadata-license-information/

I guess this is basically the same as CC0? I suppose? Anyway, just sharing this here on the ticket for posterity.