Don't allow entry of obsolete SO terms

[ahwagner]

Currently, we use obsolete SO terms in our Variant Type(s) field, which validates any entry against those terms. Consequently, when attempting to enter a newer term (e.g. SO:0001606 - amino_acid_substitution), you may instead see a listing of obsolete terms (e.g. SO:1000093 - sequence_variant_causing_amino_acid_substitution), prompting the user to select an antiquated term instead of the preferred (current) term.