add support for custom regions, by detecting what is used in index.html (#3518)

read index.html to discover the regions used, make them the list checked
by app.js and check:config test

fixes #3504   supercedes #3506 

no config.js param required

js/defaults.js

@@ -27,7 +27,6 @@ const defaults = {
 	// (interval 30 seconds). If startup-timestamp has changed the client reloads the magicmirror webpage.
 	checkServerInterval: 30 * 1000,
 	reloadAfterServerRestart: false,
-	allowCustomModulePositions: false,
 
 	modules: [
 		{