Add a web app manifest

[Glodenox]

A web app manifest would allow people to install the page as an app. Special care should be taken about caching the files though.

More details: https://developer.mozilla.org/en-US/docs/Web/Manifest

Already done: It makes sense to want to go fullscreen with the chat monitor and to provide it as a web app. To achieve that, we can provide a button to go fullscreen and a manifest file to allow the installation like a web app.

[Glodenox]

Added support for fullscreen. Not yet certain about the manifest file, as it would require the creation of an icon for the chat monitor.