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/components/_media.less

@@ -0,0 +1,13 @@
+// Captions
+.wp-caption {
+  &:extend(.thumbnail all);
+}
+
+.wp-caption-text {
+    &:extend(.thumbnail .caption);
+}
+
+// Gallery shortcode
+.gallery-row {
+  padding: (@line-height-computed / 2) 0;
+}