web: add ability to redirect HTTP traffic to HTTPS

Useful if you're running this setup directly on the Internet, with a
Let's Encrypt certificate.

env.example

@@ -121,3 +121,6 @@ JIGASI_PORT_MAX=20050
 
 # Disable HTTPS. This can be useful if TLS connections are going to be handled outside of this setup.
 #DISABLE_HTTPS=1
+
+# Redirects HTTP traffic to HTTPS. Only works with the standard HTTPS port (443).
+#ENABLE_HTTP_REDIRECT=1