Merge branch 'add-yarn-clean-step' into 'master'

Add yarn clean step, to delete production compiled assets

See merge request gitlab-org/gitlab-ce!19212

doc/development/new_fe_guide/tips.md

 # Tips
 
-> TODO: Add tips
+## Clearing production compiled assets
+
+To clear production compiled assets created with `yarn webpack-prod` you can run:
+
+```
+yarn clean
+```

package.json

 {
   "private": true,
   "scripts": {
+    "clean": "rm -rf public/assets tmp/cache/*-loader",
     "dev-server": "nodemon -w 'config/webpack.config.js' --exec 'webpack-dev-server --config config/webpack.config.js'",
     "eslint": "eslint --max-warnings 0 --ext .js,.vue .",
     "eslint-fix": "eslint --max-warnings 0 --ext .js,.vue --fix .",