Rails yanked out observers many many years ago, instead the functionality
was yanked out to a gem that is very lightly maintained.
For example: if we want to upgrade to rails 5 there is no published gem
Internally the usage of observers had quite a few problem.
The series of refactors renamed a bunch of classes to give us more clarity
and removed some magic.
* 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.
FIX: keep the "uploading..." indicator until the server replies via the MessageBus
FIX: text was disapearing when uploading an avatar
PERF: always use a region for S3 (defaults to 'us-east-1')
FEATURE: ApplyCDN middleware when using S3
FIX: use the same pattern to store files on S3 and locally
PERF: keep a local cache of uploads when generating thumbnails
FEATURE: migrate_to_s3 rake task
Like that we can have code that works on multiple Rails versions, and we
dont need to mix a new method on Kernel.
Also, this makes easier to have multiple versions.
For instance, before master was 4.2, which is not the case anymore, so
on the code we should check versions and not Environment variables
