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)

less/common/bootstrap/component-animations.less

@@ -0,0 +1,33 @@
+//
+// Component animations
+// --------------------------------------------------
+
+// Heads up!
+//
+// We don't use the `.opacity()` mixin here since it causes a bug with text
+// fields in IE7-8. Source: https://github.com/twbs/bootstrap/pull/3552.
+
+.fade {
+  opacity: 0;
+  .transition(opacity .15s linear);
+  &.in {
+    opacity: 1;
+  }
+}
+
+.collapse {
+  display: none;
+
+  &.in      { display: block; }
+  tr&.in    { display: table-row; }
+  tbody&.in { display: table-row-group; }
+}
+
+.collapsing {
+  position: relative;
+  height: 0;
+  overflow: hidden;
+  .transition-property(~"height, visibility");
+  .transition-duration(.35s);
+  .transition-timing-function(ease);
+}