Make AddIndexToProjectAuthorizations reversable

Don't try to create the index if it already exist. It's possible that a
previous rollback did not remove the index since the index is not
dropped on MySQL because MySQL would also drop the foreign key in that
case, see f67d8eb1.
Signed-off-by: Rémy Coutable <remy@rymai.me>

db/migrate/20170130204620_add_index_to_project_authorizations.rb

@@ -6,7 +6,9 @@ class AddIndexToProjectAuthorizations < ActiveRecord::Migration
   disable_ddl_transaction!
 
   def up
-    add_concurrent_index(:project_authorizations, :project_id)
+    unless index_exists?(:project_authorizations, :project_id)
+      add_concurrent_index(:project_authorizations, :project_id)
+    end
   end
 
   def down