Memoize object class titles

For an issue with around 200 notes this cuts down timings by around 150
milliseconds.

lib/banzai/filter/abstract_reference_filter.rb

@@ -22,6 +22,10 @@ module Banzai
         @data_reference ||= "data-#{object_name.dasherize}"
       end
 
+      def self.object_class_title
+        @object_title ||= object_class.name.titleize
+      end
+
       # Public: Find references in text (like `!123` for merge requests)
       #
       #   AnyReferenceFilter.references_in(text) do |match, id, project_ref, matches|
@@ -53,6 +57,10 @@ module Banzai
         self.class.object_sym
       end
 
+      def object_class_title
+        self.class.object_class_title
+      end
+
       def references_in(*args, &block)
         self.class.references_in(*args, &block)
       end
@@ -206,7 +214,7 @@ module Banzai
       end
 
       def object_link_title(object)
-        "#{object_class.name.titleize}: #{object.title}"
+        "#{object_class_title}: #{object.title}"
       end
 
       def object_link_text(object, matches)