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/modules/_accessibility.scss

@@ -35,6 +35,6 @@
 }
 
 /* Do not show the outline on the skip link target. */
-#content[tabindex="-1"]:focus {
+#primary[tabindex="-1"]:focus {
 	outline: 0;
 }

sass/modules/_clearings.scss

@@ -1,18 +0,0 @@
-.clear,
-.entry-content,
-.comment-content,
-.site-header,
-.site-content,
-.site-footer {
-
-	&::before,
-	&::after {
-
-		@include clearfix;
-	}
-
-	&::after {
-
-		@include clearfix-after;
-	}
-}