Refactor Gitlab::Favicon for EE

This moves the development favicon name in Gitlab::Favicon to a separate
method, allowing EE to redefine it to return a custom favicon.

lib/gitlab/favicon.rb

@@ -10,7 +10,7 @@ module Gitlab
           elsif Gitlab::Utils.to_boolean(ENV['CANARY'])
             'favicon-yellow.png'
           elsif Rails.env.development?
-            'favicon-blue.png'
+            development_favicon
           else
             'favicon.png'
           end
@@ -18,6 +18,12 @@ module Gitlab
         ActionController::Base.helpers.image_path(image_name, host: host)
       end
 
+      def development_favicon
+        # This is a separate method so that EE can return a different favicon
+        # for development environments.
+        'favicon-blue.png'
+      end
+
       def status_overlay(status_name)
         path = File.join(
           'ci_favicons',