Merge branch 'issue_12658' into 'master'

Check if index exists before adding it

I got an error when updating GDK because it already exists.

[ci skip]

See merge request !3335

db/migrate/20160320204112_index_namespaces_on_visibility_level.rb

 class IndexNamespacesOnVisibilityLevel < ActiveRecord::Migration
   def change
+    unless index_exists?(:namespaces, :visibility_level)
       add_index :namespaces, :visibility_level
     end
+  end
 end