Handle HGV/DCLP numbers entered in PN ID "advanced create" field

[ryanfb]

If a user enters e.g. "62766" in the PN ID field, this causes an error e.g.:

NoMethodError: undefined method `values' for nil:NilClass
/PROJECT_ROOT/app/models/publication.rb:129→ populate_identifiers_from_identifiers
/PROJECT_ROOT/app/controllers/publications_controller.rb:61→ create

We should check by prepending digit-only strings with papyri.info/hgv/ and papyri.info/dclp/ to see if we get results that way, and use those results if so. If not, we should return an error message to the user informing them to use a papyri.info string.