Use CSS animations to show delayed loading spinner

[AntonKhorev]

Similarly to #5307, we can avoid scheduling/unscheduling js events to show spinners in the richtext preview #5311 and the sidebar loader:

There's a delay to avoid blinking on fast loads, that delay can be implemented using a css animation.

And we also can stop using vendor/assets/jquery/jquery.timers.js which does some ill-advised things like listening to unload.

[tomhughes]

Looks good to me, thanks.