(css) Minify CSS using clean-css

https://github.com/jakubpawlowicz/clean-css
https://github.com/gruntjs/grunt-contrib-cssmin

Fixes #3714

UI/WebServerResources/package.json

@@ -11,6 +11,7 @@
     "grunt-contrib-jshint": ">=0.11.2",
     "grunt-contrib-uglify": ">=0.9",
     "grunt-contrib-watch": ">=0.5.3",
+    "grunt-contrib-cssmin": ">=1.0.0",
     "grunt-ng-annotate": ">=0.10.0",
     "grunt-postcss": ">=0.6.0",
     "grunt-sass": ">=0.18.1",