web: Modernize with Bower, Bootstrap, LESS

The page layout has been visually modified to add a menu on the left
side. Bootstrap framework makes it work responsively on mobiles.

Some of the main HTML elements have been changed to HTML5 elements.

All the pages have also been reworked.

Signed-off-by: db0 <db0company@gmail.com>
Signed-off-by: Timothy Gu <timothygu99@gmail.com>
Signed-off-by: Clément Bœsch <u@pkh.me>
web: announce new website
web/README: update htdocs paths
web: put default.css back, fate.ffmpeg.org still uses it

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>