Fix #4359: Allow builders to move small (artist) tags manually.

Allow builders to approve artist alias BURs. The BUR must contain only
artist aliases or mass updates and each artist must have less than 100
posts.

app/logical/bulk_update_request_processor.rb

@@ -117,6 +117,22 @@ class BulkUpdateRequestProcessor
     []
   end
 
+  def is_tag_move_allowed?
+    tokens.all? do |type, *args|
+      case type
+      when :create_alias
+        BulkUpdateRequestProcessor.is_tag_move_allowed?(args[0], args[1])
+      when :mass_update
+        lhs = PostQueryBuilder.new(args[0])
+        rhs = PostQueryBuilder.new(args[1])
+
+        lhs.is_simple_tag? && rhs.is_simple_tag? && BulkUpdateRequestProcessor.is_tag_move_allowed?(args[0], args[1])
+      else
+        false
+      end
+    end
+  end
+
   def to_dtext
     tokens.map do |token|
       case token[0]
@@ -132,5 +148,15 @@ class BulkUpdateRequestProcessor
     end.join("\n")
   end
 
+  private
+
+  def self.is_tag_move_allowed?(antecedent_name, consequent_name)
+    antecedent_tag = Tag.find_by_name(Tag.normalize_name(antecedent_name))
+    consequent_tag = Tag.find_by_name(Tag.normalize_name(consequent_name))
+
+    (antecedent_tag.blank? || antecedent_tag.empty? || (antecedent_tag.artist? && antecedent_tag.post_count <= 100)) &&
+    (consequent_tag.blank? || consequent_tag.empty? || (consequent_tag.artist? && consequent_tag.post_count <= 100))
+  end
+
   memoize :tokens
 end