Change returnUrl behaviur

[skodapetr]

As of now the returnUrl make post and follow Location header. We need to change this to work in a way a POST fith form works, i.e. the response of the POST is shown as HTML. We can test this by posting the data to the same adress the forms are running, the page should be reloaded with the posted data.

We can use something like: https://stackoverflow.com/questions/19064352/how-to-redirect-through-post-method-using-javascript/27766998

[skodapetr]

@jakubklimek We should now be able to post data back to the form and see the data reloaded.