Implement redesign, refactor everything

- Write CSS for everything, update templates.
- Refactor discussion view. Stream is split into two components
(content and scrubber) which have their own responsibilities.
- Extract pane functionality into a mixin.
- Implement global “back button” system. You give a “paneable” target
to the application controller, the back button will modulate its
pane-related properties as necessary, and call an action when the
button is clicked.
- Extract welcome-hero into its own component.
- Lots of other general improvements/refactoring. The code is quite
well-commented so take a look!

ember/app/components/back-button.js

@@ -0,0 +1,26 @@
+import Ember from 'ember';
+
+export default Ember.Component.extend({
+	classNames: ['back-button'],
+	classNameBindings: ['active'],
+	active: Ember.computed.or('target.paneShowing', 'target.panePinned'),
+ 
+	mouseEnter: function() {
+        this.get('target').send('showPane');
+    },
+
+    mouseLeave: function() {
+        this.get('target').send('hidePane');
+    },
+
+    actions: {
+    	back: function() {
+    		this.get('target').send('transitionFromBackButton');
+            this.set('target', null);
+    	},
+    	togglePinned: function() {
+            this.get('target').send('togglePinned');
+    	}
+    }
+
+});