Contact/demo forms handler for static websites (program)

 hugo-mx-gateway provides a RESTful POST endpoint for static
 contact/demo request pages. It's a simple, yet a powerful tool built for
 this only-designated purpose, e.g. for use with static sites built
 using Hugo.
 .
 hugo-mx-gateway is built upon a simple request handling workflow:
 .
  * Create an HTML form with a POST action pointing towards the hugo-mx-
    gateway service.
  * For each user request, hugo-mx-gateway automatically retrieves
    information submitted by the user (email, subject, message details...),
    then generates and sends a templated email (based on Go Template to the
    user-provided email address, while bcc'ing a copy of that email to an
    address that you can define for internal tracking and follow up.
  * Once a request is processed (upon success or failure), hugo-mx-gateway
    handles the reply back towards the calling static page by redirecting
    the browser to the origin page with additional URL parameters
    describing the completion status of the processing (e.g.
    /contact.html?status=success&message=request%20submitted). The
    parameters can then be easily retrieved and shown to the user, e.g.
    with a few lines of Javascript within the static page.