Indexing: add field (or tag?) for preservation status

[bnewbold]

A feature difference between scholar and fatcat is that fatcat displays non-IA preservation status. As a first step towards supporting this in scholar, we could/should show "Keepers" preservation status for individual works.
This could include the specific keepers (LOCKSS, Portico, Hathitrust, etc), or just indicate "in at least one keeper". It could link directly to the keeper's registry on issn.org (eg, https://keepers.issn.org/?q=api/search&search[]=MUST=allissn=0958-1596&search[]=MUST_EXIST=keepers), or to fatcat, or just be a visual label (like the other tags are currently).