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/hide-text.less

@@ -0,0 +1,21 @@
+// CSS image replacement
+//
+// Heads up! v3 launched with with only `.hide-text()`, but per our pattern for
+// mixins being reused as classes with the same name, this doesn't hold up. As
+// of v3.0.1 we have added `.text-hide()` and deprecated `.hide-text()`.
+//
+// Source: https://github.com/h5bp/html5-boilerplate/commit/aa0396eae757
+
+// Deprecated as of v3.0.1 (will be removed in v4)
+.hide-text() {
+  font: ~"0/0" a;
+  color: transparent;
+  text-shadow: none;
+  background-color: transparent;
+  border: 0;
+}
+
+// New mixin to use as of v3.0.1
+.text-hide() {
+  .hide-text();
+}