Update sprockets. (#4167)

* Update sass-rails.

* FIX: Tilt dependency has been removed from Ember::Handlebars::Template.

* Update `DiscourseIIFE` to new Sprockets API.

* `Rails.application.assets` returns `nil` in production.

* Move sprockets-rails out of the assets group.

* Pin ember-rails to 0.18.5 which works with Sprockets 3.x.

* Update sprockets to 3.6.0.

* Make `DiscourseSassCompiler` work with Sprockets 3.

* Use `Sass::Rails::SassImporterGlobbing` instead of haxxing our own.

* Moneky patch so that we don't add dependencies for our custom css.

* FIX: Missing class.

* Upgrade ember-handlebars-template.

* FIX: require path needs to share the same root as the folder's path.

* Bump discourse-qunit-rails.

* Update ember-template-compiler.js to 1.12.2.

* `prepend` is private in Ruby 2.0.0.

lib/freedom_patches/resolve.rb

@@ -0,0 +1,19 @@
+# sass-rails expects an actual file to exists when calling `@import`. However,
+# we don't actually create the files for our special imports but rather inject
+# them dynamically.
+module Discourse
+  module Sprockets
+    module Resolve
+      def resolve(path, options = {})
+        return [path, []] if DiscourseSassImporter.special_imports.has_key?(File.basename(path, '.scss'))
+        super
+      end
+    end
+  end
+end
+
+module Sprockets
+  class Base
+    prepend Discourse::Sprockets::Resolve
+  end
+end