made older listed_taxa maintain primary status when newer listed_taxa are created

I changed the name of some methods to make it clear they deal with listed_taxa and not taxa, fixed the bug I mentioned where the delayed job was setting primary to false, made it so that older listed_taxa maintain primary status when newer listed_taxa are created, and removed the assign_primary tool which isn't necessary (primary_listing default is true) but made a new script (in the migration) that makes all but one listed_taxa not-primary when there are multiple listed_taxa