Add "Requestable" flag to item data

[saverkamp]

We will need to calculate whether an item result can display a request button. We can either do this on the fly in the client or on serialization and store it as a boolean in the item data. If we're sure on the latter, we need to add a property and serialization logic.

[saverkamp]

Added nypl:requestable property with a range of xsd:boolean to Discovery Properties and mapping document.