Add access checks to datasources

I don't know why we don't have that yet, but there should be a way to check access for result items that aren't entities – i.e., a checkItemAccess() method on the datasource plugin itself. (We only ever need to check for "view" access, I'm pretty sure, so that would just need one parameter, the item.)

Then, we could also use that in the Views query plugin for applying access checks.