minor modifications to homepage

The highlight: rewrote the layout.jade file in jade only.

Also made little enhancements on the way. Very slightly changed the sign-in and sign-up modals. Created `stylesheets` and `scripts` blocks in layout so views that extend the layout can add their own stylesheets or scripts. In the previous commits, I added latest jQuery, Bootstrap and Flat-UI less and js files. The rewritten layout uses those instead of the old ones.

TL;DR there's nothing considerably different in the UI, but there are a few unnoticeable structural changes.