Configure User Invite as a plugin

[fvanderflier]

Currently we are checking if the email service is registered and if it is not we're just not sending an email.

We need to check if this service is available and tell the frontend and if not the button text should change to Create User.

          if (_emailService is null)
            {
                return Ok(new
                {
                    message = "Invitation prepared, but email service is not configured.",
                });
            }

Let's create an endpoint called Plugins and return the state of the DI container for our regular Interfaces.