Undoes skolem generation for creators implemented in 7fc4986e

Since we're no longer storing creators as individual IDB objects, they
don't need an ID. This makes these blank nodes line up with the data
model as well.
Preserve label text when parsing AD/BC dates

Before, we had been normalizing them to one form (AD instead of A.D. or
a.d.e.,  for example). This preserves the content of the original.