move CSS build to webpack (#9983)

- added new 'make webpack' target
- deprecated 'make js' and 'make css'
- extend webpack config to load the less files
- updated docs

I had to rename the source file of `arc-green.less` to avoid generating
a useless JS entrypoint via webpack-fix-style-only-entries which would
not work with different source/destination filenames. I hear that there
should be cleaner solutions possible once we upgrade to Webpack 5.

Co-authored-by: zeripath <art27@cantab.net>

.drone.yml

@@ -15,7 +15,7 @@ steps:
     pull: always
     image: node:10 # this step is kept at the lowest version of node that we support
     commands:
-      - make css js
+      - make webpack
 
   - name: build-without-gcc
     pull: always