Makefile changes for Windows and easier development (#6103)

* Added Go Path and node_modules to PATH
* Uses npx now for generate-stylesheets
* Uses `go env GOPATH` to calculate adding GOPATH/bin to PATH
* Added note about installing Node 8.0+ to generate stylesheets
* Added preferred Node version to CONTRIBUTING.md

docs/content/doc/advanced/hacking-on-gitea.en-us.md

@@ -138,6 +138,8 @@ make vet lint misspell-check
 
 ### Updating the stylesheets
 
+To generate the stylsheets, you will need [Node.js](https://nodejs.org/) at version 8.0 or above.
+
 At present we use [less](http://lesscss.org/) and [postcss](https://postcss.org) to generate our stylesheets. Do
 **not** edit the files in `public/css/` directly as they are generated from
 `lessc` from the files in `public/less/`.