[FIX] web app manifest errors as reported by Chrome DevTools (#9991)

@RocketChat/core 

Closes #9990

This pull request properly brings the web app manifest up-to-date to the current specification and resolves the incorrect start_url origin and the path to the broken images as seen in this screenshot.

This resolves the unwanted and incorrect behavior experienced in both Google Chrome and Mozilla Firefox for Android.

![image](https://user-images.githubusercontent.com/643503/36929663-052fe33c-1e49-11e8-8a68-3d8e666d30e8.png)

public/images/manifest.json

@@ -1,17 +1,19 @@
 {
 	"name": "Rocket.Chat",
+	"short_name": "Rocket.Chat",
+	"background_color": "#fff",
 	"icons": [
 		{
-			"src": "assets\/favicon_192.png",
+			"src": "/assets/favicon_192.png",
 			"sizes": "192x192",
-			"type": "image\/png"
+			"type": "image/png"
 		},
 		{
-			"src": "assets\/favicon_512.png",
+			"src": "/assets/favicon_512.png",
 			"sizes": "512x512",
-			"type": "image\/png"
+			"type": "image/png"
 		}
 	],
-	"start_url": "https:\/\/rocket.chat\/home",
+	"start_url": "/home?homescreen",
 	"display": "standalone"
 }