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/labels.less

@@ -0,0 +1,64 @@
+//
+// Labels
+// --------------------------------------------------
+
+.label {
+  display: inline;
+  padding: .2em .6em .3em;
+  font-size: 75%;
+  font-weight: bold;
+  line-height: 1;
+  color: @label-color;
+  text-align: center;
+  white-space: nowrap;
+  vertical-align: baseline;
+  border-radius: .25em;
+
+  // Add hover effects, but only for links
+  a& {
+    &:hover,
+    &:focus {
+      color: @label-link-hover-color;
+      text-decoration: none;
+      cursor: pointer;
+    }
+  }
+
+  // Empty labels collapse automatically (not available in IE8)
+  &:empty {
+    display: none;
+  }
+
+  // Quick fix for labels in buttons
+  .btn & {
+    position: relative;
+    top: -1px;
+  }
+}
+
+// Colors
+// Contextual variations (linked labels get darker on :hover)
+
+.label-default {
+  .label-variant(@label-default-bg);
+}
+
+.label-primary {
+  .label-variant(@label-primary-bg);
+}
+
+.label-success {
+  .label-variant(@label-success-bg);
+}
+
+.label-info {
+  .label-variant(@label-info-bg);
+}
+
+.label-warning {
+  .label-variant(@label-warning-bg);
+}
+
+.label-danger {
+  .label-variant(@label-danger-bg);
+}