add theater mode

[xDashh]

theater mode like on dgg/twitch was requested.
Should mainly hide the top-bar when watching on the site, chat optional, viewer stats not sure.

Implementation probably via some hotkey or user (profile) preference (once it's easier to add those code-wise with persistence...).

[kenany]

A setting in profile would be the easiest to implement, but arguably the worse user experience.

This will need a keyboard hotkey and/or an icon/button on the page. Should probably persist across streams as well.

[JoannesJ]

A bit different, but I took a look at simply triggering full-screen on the element containing the stream and chat.
It's quite comfy.
Here is a snippet to take a look, it creates an element to click in the navbar:

(function(){
	const nav = document.querySelector(".navbar-nav");
	const li = document.createElement("li");
	const a = document.createElement("a");
	li.addEventListener("click", () => { 
		document.querySelector(".header").nextSibling.requestFullscreen();
	} );
	a.textContent = "Theater";
	li.appendChild( a );
	nav.appendChild( li );
})();