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
 

app/models/tag_implication.rb

@@ -7,6 +7,7 @@ class TagImplication < TagRelationship
   validate :absence_of_transitive_relation
   validate :antecedent_is_not_aliased
   validate :consequent_is_not_aliased
+  validate :tag_categories_are_compatible
   validate :has_wiki_page, on: :request
 
   concerning :HierarchyMethods do
@@ -93,6 +94,12 @@ class TagImplication < TagRelationship
       end
     end
 
+    def tag_categories_are_compatible
+      if antecedent_tag.category != consequent_tag.category
+        errors[:base] << "Can't imply a #{antecedent_tag.category_name.downcase} tag to a #{consequent_tag.category_name.downcase} tag"
+      end
+    end
+
     def has_wiki_page
       if !antecedent_tag.empty? && antecedent_wiki.blank?
         errors[:base] << "'#{antecedent_name}' must have a wiki page"