Add start URL to manifest.json

Adds the last criteria to make the application an installable PWA on mobile.

static/manifest.json

@@ -5,6 +5,7 @@
 	"background_color": "#1f1f1f",
 	"description": "An alternative private front-end to Reddit",
 	"theme_color": "#1f1f1f",
+	"start_url": "/",
 	"icons": [
 		{
 			"src": "logo.png",
@@ -20,4 +21,4 @@
 			"sizes": "32x32"
 		}
 	]
-}
+}