web: Modernize with Bower, Bootstrap, LESS

The Makefile now installs external dependencies using Bower, such as
the web framework Bootstrap and the LESS CSS compiler, then compiles
the CSS files from the LESS sources before generating the HTML pages.

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.