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/bootstrap.less

@@ -0,0 +1,50 @@
+// Core variables and mixins
+@import "variables.less";
+@import "mixins.less";
+
+// Reset and dependencies
+@import "normalize.less";
+@import "print.less";
+@import "glyphicons.less";
+
+// Core CSS
+@import "scaffolding.less";
+@import "type.less";
+@import "code.less";
+@import "grid.less";
+@import "tables.less";
+@import "forms.less";
+@import "buttons.less";
+
+// Components
+@import "component-animations.less";
+@import "dropdowns.less";
+@import "button-groups.less";
+@import "input-groups.less";
+@import "navs.less";
+@import "navbar.less";
+@import "breadcrumbs.less";
+@import "pagination.less";
+@import "pager.less";
+@import "labels.less";
+@import "badges.less";
+@import "jumbotron.less";
+@import "thumbnails.less";
+@import "alerts.less";
+@import "progress-bars.less";
+@import "media.less";
+@import "list-group.less";
+@import "panels.less";
+@import "responsive-embed.less";
+@import "wells.less";
+@import "close.less";
+
+// Components w/ JavaScript
+@import "modals.less";
+@import "tooltip.less";
+@import "popovers.less";
+@import "carousel.less";
+
+// Utility classes
+@import "utilities.less";
+@import "responsive-utilities.less";