Update jQuery to 1.11.1
Use Gulp to process SASS etc; upgrade Bootstrap.

This is a horrible mess, and not yet working.

Most of the way through using Gulp to process SASS/CSS and JS files.
But it doesn't yet put the correct cache-busting filenames into the HTML
templates.

Also, upgrade Bootstrap to 3.3.1, which I did before I realised I hadn't
committed all the previou stuff. Ugh.