Big front-end asset/filestructure refactor

- Extract shared Ember components into a “flarum-common” ember-cli
addon. This can be used by both the forum + admin Ember apps, keeping
things DRY
- Move LESS styles into their own top-level directory and do a similar
thing (extract common styles)
- Add LESS/JS compilation and versioning to PHP (AssetManager)
- Set up admin entry point

(Theoretical) upgrade instructions:
- Delete everything in [app_root]/public
- Set up tooling in forum/admin Ember apps (npm install/update, bower
install/update) and then build them (ember build)
- php artisan vendor:publish
- Upgrade flarum/flarum repo (slight change in a config file)
- If you need to trigger a LESS/JS recompile, delete the .css/.js files
in [app_root]/public/flarum. I set up LiveReload to do this for me when
I change files in less/ or ember/

Todo:
- Start writing admin app!
- Remove bootstrap/font-awesome from repo and instead depend on their
composer packages? Maybe? (Bower is not an option here)

ember/admin/tests/integration/index-test.js

@@ -0,0 +1,48 @@
+import Ember from "ember";
+import { test } from 'ember-qunit';
+import startApp from '../helpers/start-app';
+var App;
+
+module('Index', {
+  setup: function() {
+    App = startApp();
+  },
+  teardown: function() {
+    Ember.run(App, App.destroy);
+  }
+});
+
+test('Discussion list loading', function() {
+  expect(3);
+  visit('/').then(function() {
+    equal(find('.discussions-list').length, 1, 'Page contains list of discussions');
+    equal(find('.discussions-list li').length, 20, 'There are 20 discussions in the list');
+
+    click('.control-loadMore').then(function() {
+      equal(find('.discussions-list li').length, 40, 'There are 40 discussions in the list');
+    });
+  });
+});
+
+test('Discussion list sorting', function() {
+  expect(1);
+  visit('/').then(function() {
+    fillIn('.control-sort select', 'replies').then(function() {
+      var discussions = find('.discussions-list li');
+      var good = true;
+      var getCount = function(item) {
+        return parseInt(item.find('.count strong').text());
+      };
+      var previousCount = getCount(discussions.eq(0));
+      for (var i = 1; i < discussions.length; i++) {
+        var count = getCount(discussions.eq(i));
+        if (count > previousCount) {
+          good = false;
+          break;
+        }
+        previousCount = count;
+      }
+      ok(good, 'Discussions are listed in order of reply count');
+    });
+  });
+});