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/grid.less

@@ -0,0 +1,84 @@
+//
+// Grid system
+// --------------------------------------------------
+
+
+// Container widths
+//
+// Set the container width, and override it for fixed navbars in media queries.
+
+.container {
+  .container-fixed();
+
+  @media (min-width: @screen-sm-min) {
+    width: @container-sm;
+  }
+  @media (min-width: @screen-md-min) {
+    width: @container-md;
+  }
+  @media (min-width: @screen-lg-min) {
+    width: @container-lg;
+  }
+}
+
+
+// Fluid container
+//
+// Utilizes the mixin meant for fixed width containers, but without any defined
+// width for fluid, full width layouts.
+
+.container-fluid {
+  .container-fixed();
+}
+
+
+// Row
+//
+// Rows contain and clear the floats of your columns.
+
+.row {
+  .make-row();
+}
+
+
+// Columns
+//
+// Common styles for small and large grid columns
+
+.make-grid-columns();
+
+
+// Extra small grid
+//
+// Columns, offsets, pushes, and pulls for extra small devices like
+// smartphones.
+
+.make-grid(xs);
+
+
+// Small grid
+//
+// Columns, offsets, pushes, and pulls for the small device range, from phones
+// to tablets.
+
+@media (min-width: @screen-sm-min) {
+  .make-grid(sm);
+}
+
+
+// Medium grid
+//
+// Columns, offsets, pushes, and pulls for the desktop device range.
+
+@media (min-width: @screen-md-min) {
+  .make-grid(md);
+}
+
+
+// Large grid
+//
+// Columns, offsets, pushes, and pulls for the large desktop device range.
+
+@media (min-width: @screen-lg-min) {
+  .make-grid(lg);
+}