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/style.scss

@@ -32,7 +32,6 @@ Nicolas Gallagher and Jonathan Neal https://necolas.github.io/normalize.css/
 	## Menus
 # Accessibility
 # Alignments
-# Clearings
 # Widgets
 # Content
 	## Posts and pages
@@ -80,11 +79,6 @@ Nicolas Gallagher and Jonathan Neal https://necolas.github.io/normalize.css/
 --------------------------------------------------------------*/
 @import "modules/alignments";
 
-/*--------------------------------------------------------------
-# Clearings
---------------------------------------------------------------*/
-@import "modules/clearings";
-
 /*--------------------------------------------------------------
 # Widgets
 --------------------------------------------------------------*/