Fixes element-hq/element-web#9303

We have to track our own onHover for this, and out of safety we ensure that exactly 1 room is hovered at a time.

Fixes element-hq/element-web#8714
Fixes element-hq/element-web#8890
Fixes element-hq/element-web#9034
Fixes element-hq/element-web#8954

This turned out to be much more complicated than it needed to be. We use an IndicatorScrollbar to do all the math for us and some minor changes have been made so it can flag left/right overflow. The complicated part is the css changes which make the gradients work: unlike the RoomSubList, we have to calculate the offset of the indicators (gradients) on our own because position:sticky doesn't work horizontally.

The changes to the css (well, mostly pointer-events:none) make it so the gradient doesn't interfere with the room avatars. 

9034 and 8954 are fixed by this because they represent an overflow-x:none style breakage where browsers won't let you scroll without a scrollbar. The gradient offset problem is also demonstrated in 8954.