DEV: explicitly require Rails components

`rails/all` includes too much stuff per: https://github.com/rails/rails/blob/master/railties/lib/rails/all.rb

This commit makes it explicit what pieces of Rails Discourse depends on.

Previously the LoadError was protecting us and we were excluding components,
using the Gemfile, this method ensures that even if we add `rails` meta gem
as a dependency only the parts of Rails Discourse uses will be used.

config/application.rb

@@ -14,7 +14,11 @@ rescue
 end
 
 require File.expand_path('../boot', __FILE__)
-require 'rails/all'
+require 'active_record/railtie'
+require 'action_controller/railtie'
+require 'action_view/railtie'
+require 'action_mailer/railtie'
+require 'sprockets/railtie'
 
 # Plugin related stuff
 require_relative '../lib/discourse_event'