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/responsive-embed.less

@@ -0,0 +1,35 @@
+// Embeds responsive
+//
+// Credit: Nicolas Gallagher and SUIT CSS.
+
+.embed-responsive {
+  position: relative;
+  display: block;
+  height: 0;
+  padding: 0;
+  overflow: hidden;
+
+  .embed-responsive-item,
+  iframe,
+  embed,
+  object,
+  video {
+    position: absolute;
+    top: 0;
+    left: 0;
+    bottom: 0;
+    height: 100%;
+    width: 100%;
+    border: 0;
+  }
+}
+
+// Modifier class for 16:9 aspect ratio
+.embed-responsive-16by9 {
+  padding-bottom: 56.25%;
+}
+
+// Modifier class for 4:3 aspect ratio
+.embed-responsive-4by3 {
+  padding-bottom: 75%;
+}