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/helpers/fa-icon.js

@@ -1,6 +1,6 @@
 import Ember from 'ember';
 
 export default Ember.Handlebars.makeBoundHelper(function(icon, options) {
-	return new Handlebars.SafeString('<i class="fa fa-icon fa-'+icon+' '+(options.hash.class || '')+'"></i>');
+	return new Handlebars.SafeString('<i class="fa fa-fw fa-'+icon+' '+(options.hash.class || '')+'"></i>');
 });