Fix the backported migration

For those who upgraded to a version that included the previous
backport implementation with migration 20190311132500.

db/migrate/20180115094742_add_default_project_creation_setting.rb

@@ -6,10 +6,14 @@ class AddDefaultProjectCreationSetting < ActiveRecord::Migration[4.2]
   disable_ddl_transaction!
 
   def up
+    unless column_exists?(:application_settings, :default_project_creation)
       add_column_with_default(:application_settings, :default_project_creation, :integer, default: 2)
     end
+  end
 
   def down
+    if column_exists?(:application_settings, :default_project_creation)
       remove_column(:application_settings, :default_project_creation)
     end
+  end
 end