Add common folder, add grid component, remove home placeholder

* New `common/` folder for `_variables` and `_global`
* Move grid styles to new `_grid` component
* Remove `layouts/pages/_home`

assets/styles/layouts/_general.less

@@ -1,10 +0,0 @@
-// Content wrapper
-.wrap { }
-
-// Main content area
-.main {
-  .make-sm-column(@main-sm-columns);
-  .sidebar-primary & {
-    .make-sm-column(@main-sm-columns - @sidebar-sm-columns);
-  }
-}

assets/styles/layouts/_sidebar.less

@@ -1,3 +0,0 @@
-.sidebar {
-  .make-sm-column(@sidebar-sm-columns);
-}