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
Install Firefox Webapp from local checkout

Ask Firefox to install the currently running copy of glowing bear
instead of the copy from torhve.github.io

This also fixes a bug when glowing bear is not running from a
subdirectory (or one other than /glowing-bear) which would lead to a 404
because of torhve.github.io/original_subdir being loaded instead of
/glowing-bear.

Note that installing to a different subdirectory than /glowing-bear will
lead to the icons not being found due to the fact that all icon paths
inside manifest.webapp have to be absolute.
Merge pull request #174 from tribut/webapp

Install Firefox Webapp from local checkout
Merge pull request #173 from lorenzhs/mobilescroll

Improve mobile behaviour
Set container to panel