Add migration to rename column light_logo -> header_logo

Also removes dark_logo which was unused

db/migrate/20160302141317_rename_header_field_on_appearrance.rb

+class RenameHeaderFieldOnAppearrance < ActiveRecord::Migration
+  def change
+    rename_column :appearances, :light_logo, :header_logo
+
+    remove_column :appearances, :dark_logo
+  end
+end