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/_general.less

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