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/mixins/paneable.js

@@ -0,0 +1,47 @@
+import Ember from 'ember';
+
+// This mixin defines a "paneable" controller - this is, one that has a
+// portion of its interface that can be turned into a pane which slides out
+// from the side of the screen. This is useful, for instance, when you have
+// nested routes (index > discussion) and want to have the parent
+// route's interface transform into a side pane when entering the child route.
+export default Ember.Mixin.create({
+	needs: ['application'],
+
+	// Whether or not the "paneable" interface element is paned.
+	paned: false,
+
+	// Whether or not the pane should be visible on screen.
+	paneShowing: false,
+	paneHideTimeout: null,
+
+	// Whether or not the pane is always visible on screen, even when the
+	// mouse is taken away.
+	panePinned: false,
+
+	actions: {
+		showPane: function() {
+			if (this.get('paned')) {
+				clearTimeout(this.get('paneHideTimeout'));
+				this.set('paneShowing', true);
+			}
+		},
+
+		hidePane: function(delay) {
+			var controller = this;
+			controller.set('paneHideTimeout', setTimeout(function() {
+            	controller.set('paneShowing', false);
+            }, delay || 250));
+		},
+		
+		togglePinned: function() {
+			this.toggleProperty('panePinned');
+		}
+	},
+
+	// Tell the application controller when we pin/unpin the pane so that
+	// other parts of the interface can respond appropriately.
+	panePinnedChanged: function() {
+		this.set('controllers.application.panePinned', this.get('paned') && this.get('panePinned'));
+	}.observes('paned', 'panePinned')
+});