Template mode, new view create_signature_template for demo

You need to define PYDOCUSIGN_TEST_TEMPLATE_ID in your environnement for
testing
Do we have to remember the template ID?
I mean:

* the envelope is actually created on DocuSign using a template ID
* then in Signature model we store signature_backend_id, which is envelope's ID
* so with signature_backend_id we can retrieve template ID, can't we?