Instead of using CSS to limit the height of package comments as
implemented in 7b13203b

 (Limit comment height to 15 lines, 2016-03-12),
use JavaScript to collapse long comments and add a link to expand them.
Clicking the same link twice results in the corresponding comment being
collapsed again.

If JavaScript is disabled, the full comments are shown (without any
possibility to collapse or expand).

Signed-off-by: Lukas Fleischer <lfleischer@archlinux.org>