BURs: don't allow implying tags from different categories.

Don't allow requests for implications between tags of different
categories. For example, don't allow character tags to imply copyright
tags.

app/models/artist.rb

@@ -192,6 +192,7 @@ class Artist < ApplicationRecord
 
         # potential race condition but unlikely
         unless TagImplication.where(:antecedent_name => name, :consequent_name => "banned_artist").exists?
+          Tag.find_or_create_by_name("artist:banned_artist") # ensure the banned_artist exists and is an artist tag.
           TagImplication.approve!(antecedent_name: name, consequent_name: "banned_artist", approver: banner)
         end