Add Identifier term to ETDs

ACCEPTANCE CRITERIA

[x] When I look at the label for the field it should read "Identifier" (Hyrax default)
[x] Attribute name is identifier (Hyrax default)
[x] Predicate should be inherited from Hyrax basic metadata (::RDF::Vocab::DC.identifier (via Hyrax::BasicMetadata)
[x] The data type is a string
[x] I should be able to add more than one identifier
[x] I should be able to remove an identifier I have added
[x] When I look at the search results page I should not see identifiers for each ETD
[x] I should not be able to facet on identifier
[x] When I look at the .ttl view I should be able to find the identifier

SPREADSHEET https://docs.google.com/spreadsheets/d/1HZpXyVM8xaZZ9mE2uzsbUmWKbCd6NYJTESOrpQeMa18/edit#gid=0

curationexperts / mahonia