Move the Mailu Docker network to a fixed subnet.

This will make network configuration and host based authentication
more robust, across different deployment platforms.
The options `RELAYNETS` and`POD_ADDRESS_RANGE` are kept for compatibility.
However, their usage have become optional.

setup/templates/steps/stack/03_expose.html

@@ -11,9 +11,9 @@ you expose it to the world.</p>
 </div>
 
 <div class="form-group">
-  <label>Subnet</label>
+  <label>Subnet of the docker network. This should not conflict with any networks to which your system is connected. (Internal and external!)</label>
   <input class="form-control" type="text" name="subnet" required pattern="^([0-9]{1,3}\.){3}[0-9]{1,3}(\/([0-9]|[1-2][0-9]|3[0-2]))$"
-  		value="192.168.0.0/24">
+    value="192.168.203.0/24">
 </div>
 
 <p>You server will be available under a main hostname but may expose multiple public