Welcome page introduced

The goal is to be able to store and load scrapbooks. This means that
there should be a page with a list of saved scrapbooks and a button for
creating new ones. This is the first step towards:

- index.{jade, css, js} is the welcome page with a ‘New scrapbook’
button that opens a new srapbook in a new tab
- scrapbook.{jade, css, js) is the actual scrapbook page