Fix scrolling on mobile devices

Previously, we were scrolling the div, not its contents, as it grew infinitely.
Thinner scrollbars and larger buttons on mobile

Give those buttons a size where there's at least a chance to hit them with a finger
Increase size of bufferlines on mobile

We had 10px+2% space on the bottom, that's pretty wasteful
Extend swipe region. Still not optimal.

It would be nice if we could scroll on the whole `content` div, but then the
open sidebar and close nicklist swipes collide
Adaptive nicklist width
Merge pull request #173 from lorenzhs/mobilescroll

Improve mobile behaviour