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/forum/hero.less

@@ -0,0 +1,45 @@
+.hero {
+  margin-top: -1px;
+  background: @fl-body-hero-bg;
+  text-align: center;
+  padding: 20px 0;
+
+  &, & a, & .close {
+    color: @fl-body-hero-color;
+  }
+  & h2 {
+    margin: 0;
+    font-size: 16px;
+    font-weight: normal;
+    line-height: 1.5em;
+  }
+}
+.welcome-hero {
+  & a, & .close {
+    opacity: 0.5;
+  }
+  & .close {
+    float: right;
+    margin-top: -10px;
+  }
+  & p {
+    margin: 5px 0 0;
+  }
+}
+@media @phone {
+  .hero {
+    & .close {
+      margin-right: -10px;
+    }
+  }
+}
+@media @tablet, @desktop, @desktop-hd {
+  .hero {
+    padding: 30px 0;
+    font-size: 14px;
+
+    & h2 {
+      font-size: 22px;
+    }
+  }
+}