This replaces the webassets build for the front-end JavaScript
and CSS with Gulp + Browserify. webassets continues to be used but
only to serve the assets, not to generate them or keep
track of dependencies.

 * Use CommonJS for all JS dependency imports rather than relying
   on Window globals. This makes dependencies more explicit
   and avoids the need to specify dependencies explicitly
   in the build tool, since they can be statically analyzed
   via the require graph.

 * Remove the webassets custom filters for Browserify and
   CleanCSS.

 * Put all generated JS/CSS/font/image assets in
   build/{images,fonts,styles,scripts}

 * Replace compass with node-sass + compass-mixins for building
   the SCSS files. We were not relying heavily on compass
   functions, many of them were used only for vendor prefixing
   which is now handled by autoprefixer, so we can remove the
   Ruby/compass dependency and substitute node-sass quite easily.