Multiple entry points can't result in a single bundle.css with a fixed filename, see
https://github.com/webpack/extract-text-webpack-plugin/issues/179
Until that's resolved, it's easier to keep the 'css' task separate in Webpack,
and have a single entry point for all CSS (bundle.scss).
Also partially reverting "Moved frontend assets into admin/ "module"",
which moved too many files: debug.css and install.css need to remain
as framework (not admin) deps. Split out into a separate `framework-css` Webpack
task in preparation for splitting off the module.
