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.
We've removed the ability to directly reference JS and CSS files
for form fields and other SilverStripe features in favour of a common bundle built by Webpack.
The logical next step is to make the framework module free of frontend dependencies,
which should simplify its operation, and avoid another time intensive "npm install" on a module.
Many of these are now inline via Webpack's url-loader plugin (https://github.com/webpack/url-loader),
and a tonne were legacy references (e.g. leftovers from ComplexTableField).
Some of these might still be used in other modules like userforms,
but we're removing support for "hotlinking" images in core - they're not an API.
