DDK-241 index embargo date

This is mainly an improvement in calculating authorizations for Components by eliminating the need to fetch the parent Item document to check its embargo date, if set (in the available attribute). Note that embargo restrictions are separate from role-based access controls b/c embargos are time-based.

FWIW, I also learned in this work that only Item and Component resources implement Ddr::Embargoable -- i.e., Collection, Attachment, and Target resources are not embargoable.