improve emoji checking performance

config/initializers/emoji.rb

+module Emoji
+  path  = "#{Rails.root}/vendor/assets/images/emoji"
+  NAMES = Dir["#{path}/*.png"].sort.map {|f| File.basename(f, '.png')}
+end

lib/gitlab/markdown.rb

@@ -128,7 +128,7 @@ module Gitlab
     #
     # Returns boolean
     def valid_emoji?(emoji)
-      File.exists?(Rails.root.join('app', 'assets', 'images', 'emoji', "#{emoji}.png"))
+      Emoji::NAMES.include? emoji
     end
 
     # Private: Dispatches to a dedicated processing method based on reference