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>

CONTRIBUTING.md

@@ -114,15 +114,7 @@ included in the next released version.
 
 ## Building Gitea
 
-Generally, the go build tools are installed as-needed in the `Makefile`.
-An exception are the tools to build the CSS, JS and images.
-
-- To build CSS and JS: Install [Node.js](https://nodejs.org/en/download/package-manager) at version 10.0 or above
-  with `npm` and then run `npm install`, `make css` and `make js`.
-- To build Images: ImageMagick, inkscape and zopflipng binaries must be
-  available in your `PATH` to run `make generate-images`.
-
-For more details on how to generate files, build and test Gitea, see the [hacking instructions](https://docs.gitea.io/en-us/hacking-on-gitea/)
+See the [hacking instructions](https://docs.gitea.io/en-us/hacking-on-gitea/).
 
 ## Code review