Refactor the css layout to use grid and flex

Remove #primary

Remove #primary, take 2

Change primary layout module to CSS grid.

Remove ::before / ::after based clearfixes. When using flex and grid, pseudo-elements are considered flex/grid elements and cause unexpected behavior. Clearfix is a tool to resolve clearing when using float-based layouts. Modern flex/grid-based layouts do not need them.

Main menu: Use flex as layout module.

Main menu cleanup.

Comments, Posts, and Post Navigation: Use flex for layout. Automatic RTL.

Remove errant test data.

- Fixed skip to content link to point to main content (#primary).
- Changed ID for <main> to #primary to preserve original separation between main and sidebar (#secondary).
- Removed superfluous ID reference to #secondary in CSS.
- Cleaned up whitespace triggering Travis errors.

Fix wpcs issues

Cleaning the rebase

Compile CSS

Remove primary div from WooCommerce too

grid-template-columns property doesn't support negative values

sass/mixins/_mixins-master.scss

@@ -11,18 +11,6 @@
 	margin-right: auto;
 }
 
-// Clearfix
-@mixin clearfix() {
-	content: "";
-	display: table;
-	table-layout: fixed;
-}
-
-// Clear after (not all clearfix need this also)
-@mixin clearfix-after() {
-	clear: both;
-}
-
 // Column width with margin
 @mixin column-width($numberColumns: 3) {
 	width: map-get($columns, $numberColumns) - ( ( $columns__margin * ( $numberColumns - 1 ) ) / $numberColumns );