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/admin/app.less

@@ -0,0 +1,23 @@
+@import "@{common-path}/config.less";
+
+@common-path: "../common";
+
+@import "@{common-path}/bootstrap.less";
+
+// We want to specify the @fa-font-path variable AFTER we import font awesome
+// so that it overrides the default definition.
+@import "@{common-path}/font-awesome/font-awesome.less";
+@fa-font-path: "/flarum/fonts";
+
+@import url(http://fonts.googleapis.com/css?family=Open+Sans:400italic,700italic,400,700,300);
+
+// Finally, with our vendor CSS loaded, we can import Flarum-specific stuff.
+@import "@{common-path}/components.less";
+@import "@{common-path}/buttons.less";
+@import "@{common-path}/badges.less";
+@import "@{common-path}/dropdowns.less";
+@import "@{common-path}/avatars.less";
+@import "@{common-path}/forms.less";
+@import "@{common-path}/alerts.less";
+@import "@{common-path}/modals.less";
+@import "@{common-path}/layout.less";