Reorganizes asset file structure

Separates assets into
- Source assets: `assets`
- Compiled assets: `dist`

Edits assets.php to reflect changes

Removes '.min' from filenames

Fixes:
- removes 'clean' from build step.

assets/styles/layouts/_sidebar.less

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