Move the Common Widths into a Sass list

Mmm, cloudy lemonade.

Makes it easier to add custom percentage widths by using a Sass list to
store them like an array that we then loop over. Keeps configuration and
customisation inside the config file, and grid building in the grid
file.

Also a little bit of housekeeping with indentation, and using Sass' native percentage function just 'cuz.