Introduce LESS & UX Improvements & build cleanup

- some UX improvements/cleanup (AGPUSH-671)
- introduced LESS compilation (AGPUSH-646)
- build cleanup (Gruntfile.js/package.json) (AGPUSH-646)
- removed unused grunt tasks

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I want to get this merged before finishing whole build refactoring / UX improvements because of replacement of CSS resources with LESS.
It should prevent future massive conflicts when someone else modifies CSS files.

I could also pick just relevant commits relevant to LESS introduction, but this is probably simpler.

The commits are fine-grained in order to reveal what was happening.