LPS-40873 Chat video implementation (v4)

Here's v4, rebased on your master and with your review and `check_sf` applied.

There's still one bug that I know of, which I shall fix soon, in which the CSS animations are retriggered on panel show, if the animation classes were already set. I need to either replace those animation classes with fixed-display classes on panel hide/show, or when their execution is finished using an event listener in plain old JavaScript. Unless you have another idea?