Move default brand title to a method

This moves the default brand title in AppearancesHelper#brand_title to a
separate method, allowing EE to redefine it without having to redefine
the entire #brand_title method.

app/helpers/appearances_helper.rb

@@ -2,7 +2,12 @@
 
 module AppearancesHelper
   def brand_title
-    current_appearance&.title.presence || 'GitLab Enterprise Edition'
+    current_appearance&.title.presence || default_brand_title
+  end
+
+  def default_brand_title
+    # This resides in a separate method so that EE can easily redefine it.
+    'GitLab Community Edition'
   end
 
   def brand_image

ee/app/helpers/ee/appearances_helper.rb

@@ -2,6 +2,8 @@
 
 module EE
   module AppearancesHelper
+    extend ::Gitlab::Utils::Override
+
     def header_message
       return unless current_appearance&.show_header?
 
@@ -17,6 +19,11 @@ module EE
       render_message(:footer_message)
     end
 
+    override :default_brand_title
+    def default_brand_title
+      'GitLab Enterprise Edition'
+    end
+
     private
 
     def render_message(field_sym, class_names = [])