Fix thank you message on order completion

Thank you message was no more displayed since it was using a wrong
params to test when order was just completed.
This solve the issue by using flash[:commerce_tracking] param
to test when to display the message. An helper method has been
added to aviod code duplication.
Use a more relavant naming and avoid using useless
'nothing special' string as content of the flash message.

Fixes #4586