Depending on libsass for all Wagtail installations was causing issues
with various operating systems, dramatically increasing installation
times, and preventing front end development adopting modern practices.
libsass has been removed as a dependency. Sass files are compiled before
release as a dependency of the `sdist` setup command. Sass compilation
is done through `gulp`. People wishing to hack on the frontend assets
should now install wagtail locally, install the node dependencies, and
run `gulp`:
$ pip install -e path/to/wagtail
$ cd path/to/wagtail
$ npm install
$ npm start
All the templates and JS files have been updated to reference the new
compiled CSS files instead of the Sass files.
Precompiled CSS for jquery-ui and similar have been moved out of the
`scss/` directory to the `css/` directory.
