Change the SASS processing pipeline to match the h and lms projects. The
purpose of this is cross-project consistency for easier maintenance,
consistent processing of SASS files from the upcoming pattern library
and to enable us to use the new SASS module system [1]

The main changes are:

 - Use Dart Sass (the canonical SASS implementation) instead of node-sass.

   This removes a native dependency which occassionally causes build
   headaches (eg. when the installed Node version changes) and will also enable
   us to make use of the new SASS module system.

 - Use a small utility module, `create-style-bundle.js`, that calls the SASS
   compiler directly instead of using gulp plugins. This simplifies
   understanding the build process by removing dependencies and is
   consistent with the h and lms projects.

   This module was copied from the hypothesis/lms repo and modified to
   add the `postcss-url` URL rewriting plugin that the previous
   gulp-sass based pipeline had.

[1] http://sass.logdown.com/posts/7858341-the-module-system-is-launched