Generalizes `within` scope for attributes field
Generalizes `within` scope for attributes field

CSS for different types of content are different:
table.table.table-striped.document.attributes
table.table.table-striped.finding_aid.attributes
table.table.table-striped.dataset.attributes

This change generalizes the within scope so that 'valid_page_content?'
method can work for verifying the attributes section of different
types of data.

ndlib/QA_tests,ndlib/QA_tests,ndlib/QA_tests