Allow use of dartsass for scss compilation

Developer testing npm install run grunt - confirm no files are changed run behat Testing scenario 1: Test if all theme CSS and Font awesome icons look ok Login as Admin Purge all caches Verify CSS styling did not change visually Verify Fontawesome icons are rendered correctly Verify the JS functionality works for: Alerts (dismiss), Buttons (remember state), Carousel (slide), Dropdown (show menu), Popover (show popover). Tooltips (show tooltip) Access to your apache logs (they use to be placed at /var/log/apache2/error.log) and check that there are no sass parse errors there. Testing scenario 2: Check logs Login as Admin Navigate to Site Administration -> Development -> Debugging Choose Developer.. at Debug messages Tick the Display debug messages checkbox Open a console showing your web server / PHP log files (they use to be placed at /var/log/apache2/access.log and /var/log/apache2/error.log) Navigate to Site Administration -> Development -> Purge Caches Navigate to the Site Homepage Wait for the page to load and rebuild caches Check  log files (access.log and error.log) doesn't contain errors.