Grid: switch to margin approach and CSS variables

Created by: MartijnCuppens

In draft, do not review yet

• Switch from paddings to left margin
  • This way the .container or .overflow-hidden are not needed as row wrappers anymore
  • Cards can be direct children of a .row
• Remove the widths from columns:
  • seperation between columns and width utilities
• Store gutter widths in variables
  • Works a little more flexible than the universal selector technique we had
• Enabled vertical gutters by default

Examples:

• Cards without wrapping divs: https://deploy-preview-30380--twbs-bootstrap.netlify.com//docs/4.3/components/card/#grid-cards
• Grid without wrapping container: https://deploy-preview-30380--twbs-bootstrap.netlify.com//docs/4.3/layout/grid/