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/mixins/responsive-visibility.less

@@ -0,0 +1,15 @@
+// Responsive utilities
+
+//
+// More easily include all the states for responsive-utilities.less.
+.responsive-visibility() {
+  display: block !important;
+  table&  { display: table !important; }
+  tr&     { display: table-row !important; }
+  th&,
+  td&     { display: table-cell !important; }
+}
+
+.responsive-invisibility() {
+  display: none !important;
+}