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/breadcrumbs.less

@@ -0,0 +1,26 @@
+//
+// Breadcrumbs
+// --------------------------------------------------
+
+
+.breadcrumb {
+  padding: @breadcrumb-padding-vertical @breadcrumb-padding-horizontal;
+  margin-bottom: @line-height-computed;
+  list-style: none;
+  background-color: @breadcrumb-bg;
+  border-radius: @border-radius-base;
+
+  > li {
+    display: inline-block;
+
+    + li:before {
+      content: "@{breadcrumb-separator}\00a0"; // Unicode space added since inline-block means non-collapsing white-space
+      padding: 0 5px;
+      color: @breadcrumb-color;
+    }
+  }
+
+  > .active {
+    color: @breadcrumb-active-color;
+  }
+}