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/ui/controls/loading-indicator.js

@@ -1,9 +1,10 @@
 import Ember from 'ember';
 
 export default Ember.Component.extend({
-	classNames: ['loading'],
+	classNames: ['loading-indicator'],
 
 	layout: Ember.Handlebars.compile(' '),
+	size: 'small',
 
 	didInsertElement: function() {
 		this.$().spin(this.get('size'));