Add Jinja2 template support for verification response (and SMS verification)

[Azrenbeth]

Currently Jinja templates are only possible for emails, not for the HTML verification_response.
(See the templates in res/ for examples of the currently supported templates)

It would be nice to support a Jinja version for consistency and to create an example template
that renders the same file as the non-jinja version and store it in res/.

This would require altering the render_GET() method in the EmailValidateCodeServlet class.

It might also be nice to add Jinja templating support for the SMS verification texts - although these
will probably be less useful.