Fix representation of unavailable titles in metadata records

[rhigman]

Not all metadata output formats appropriately handle non-ACTIVE works.

• KBART output includes all works in publisher record, irrespective of work status. There is no representation of work status in the KBART output.
• Overdrive ONIX 3.0 output correctly sets <PublishingStatus> tag based on work status, but hard-codes the complementary <ProductAvailability> tag to 20 (Available).
• Other outputs not checked but may have similar issues (e.g. Google Books ONIX 3.0 has the same <ProductAvailability> hard-coding).

This affects publishers' ability to withdraw works from distribution where required (e.g. if they become out-of-print). In response to queries from a Thoth publisher wanting to do this:

• The way to withdraw titles from OCLC is to "remove them from the ebook feed" (presumably, the full publisher KBART record, which OCLC is harvesting for this publisher).
• It is important to set the "availability code" for withdrawn titles correctly when submitting future ONIX feeds (i.e. full publisher records) to Overdrive, otherwise they may automatically get marked as available again. (Unclear whether this referred to <PublishingStatus>, <ProductAvailability> or both; seeking clarification.)

It would be fairly easy to correctly set the <ProductAvailability> tag based on Thoth work status for all ONIX outputs. However, we would need to be cautious, as not all platforms support the full set of available values for this tag (e.g. Google Books doesn't accept 09 Postponed indefinitely).

May be related to #583.

[rhigman]

Following further discussion between publisher and target platforms:

• OCLC (KBART) happy to implement manual override so that withdrawn books which continue to appear in feed are not re-added (though it would be preferable if they were removed from feed)
• Overdrive will do something similar ("perform internal updates to [publisher] account's custom ingestion to support the collection of availability information from <PublishingStatus>")

[rhigman]

A further update: although OCLC have been able to implement a workaround for the withdrawn titles continuing to appear in the KBART feed, this is not the case for ProQuest ExLibris/ProQuest KB, who also ingest the KBART. Withdrawn titles will therefore still appear in ProQuest ExLibris/ProQuest KB until this issue is fixed in code.

Other than that, the relevant works have been successfully removed from most other platforms used by this publisher, so non-KBART output formats may not require changes.