Make favicon paths configurable

The favicons were hardcoded in launch.html, so
rebranding the page required editing the template.

The paths now come from the favicons block in
assets/config.json. Missing keys keep the defaults
from launch.html.

assets/config.json.sample

@@ -6,6 +6,11 @@
   "launchTimeout": 2000,
   "backgroundImage": "path/to/backgroundimage",
   "logo": "path/to/logo",
+  "favicons": {
+    "ico": "path/to/favicon.ico",
+    "svg": "path/to/favicon.svg",
+    "appleTouch": "path/to/apple-touch-icon.png"
+  },
   "downloads": {
     "windows": "https://github.com/jitsi/jitsi-meet-electron/releases/latest/download/jitsi-meet.exe",
     "macos": "https://github.com/jitsi/jitsi-meet-electron/releases/latest/download/jitsi-meet.dmg",