PWA Manifest: Tweaks during review of PR #4430

- Updated to go through HomeController with the builder as a helper
  class.
- Extracted some reapeated items into variables in manifest.
- Updated background color to match those used by BookStack.
- Removed reference of icon.ico since its not intended to be used.
- Added tests to cover functionality.

Review of #4430

app/App/HomeController.php

@@ -140,4 +140,12 @@ class HomeController extends Controller
         $exists = $favicons->restoreOriginalIfNotExists();
         return response()->file($exists ? $favicons->getPath() : $favicons->getOriginalPath());
     }
+
+    /**
+     * Serve a PWA application manifest.
+     */
+    public function pwaManifest(PwaManifestBuilder $manifestBuilder)
+    {
+        return response()->json($manifestBuilder->build());
+    }
 }