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.less

@@ -0,0 +1,39 @@
+// Mixins
+// --------------------------------------------------
+
+// Utilities
+@import "mixins/hide-text.less";
+@import "mixins/opacity.less";
+@import "mixins/image.less";
+@import "mixins/labels.less";
+@import "mixins/reset-filter.less";
+@import "mixins/resize.less";
+@import "mixins/responsive-visibility.less";
+@import "mixins/size.less";
+@import "mixins/tab-focus.less";
+@import "mixins/text-emphasis.less";
+@import "mixins/text-overflow.less";
+@import "mixins/vendor-prefixes.less";
+
+// Components
+@import "mixins/alerts.less";
+@import "mixins/buttons.less";
+@import "mixins/panels.less";
+@import "mixins/pagination.less";
+@import "mixins/list-group.less";
+@import "mixins/nav-divider.less";
+@import "mixins/forms.less";
+@import "mixins/progress-bar.less";
+@import "mixins/table-row.less";
+
+// Skins
+@import "mixins/background-variant.less";
+@import "mixins/border-radius.less";
+@import "mixins/gradients.less";
+
+// Layout
+@import "mixins/clearfix.less";
+@import "mixins/center-block.less";
+@import "mixins/nav-vertical-align.less";
+@import "mixins/grid-framework.less";
+@import "mixins/grid.less";