[pu-2.0] Improve overall styling

[FranckKe]

Add sass support Synchronise breakpoints between responsive menu and responsive tool bar Override some bulma variables and colors Change primary color from purple to blue Darken active/hover button state Override bulma-step colors Move responsive breakpoints from 768px to 1024px Redesign tool buttons Use header-height variable in scss instead of hard-coded value Center user dropdown links (top-right when logged in) Add quick date filter buttons by year 2016, 2017, 2018 Tweak max zoom Improve text pages css

[pierre-charpentier]

I committed some indentation changes that I spotted