DEV: Upgrade Discourse to Rails 6 (#8083)

* Adjustments to pass specs on Rails 6.0.0
* Use classic autoloader instead of Zeitwerk
* Update Rails 6.0.0 deprecated methods
* Rails 6.0.0 not allowing column with integer name
* Drop freedom_patches/rails6.rb
* Default value for trigger_transactional_callbacks? is true
* Bump rspec-rails version to 4.0.0.beta2

db/migrate/20130430052751_add_topic_allowed_groups.rb

@@ -4,8 +4,8 @@ class AddTopicAllowedGroups < ActiveRecord::Migration[4.2]
   def change
     create_table :topic_allowed_groups, force: true do |t|
       # oops
-      t.integer :group_id, :integer, null: false
-      t.integer :topic_id, :integer, null: false
+      t.integer :group_id, null: false
+      t.integer :topic_id, null: false
     end
 
     add_index :topic_allowed_groups, [:group_id, :topic_id], unique: true

db/migrate/20130501105651_fix_topic_allowed_groups.rb

@@ -3,6 +3,6 @@
 class FixTopicAllowedGroups < ActiveRecord::Migration[4.2]
   def change
     # big oops
-    remove_column :topic_allowed_groups, :integer
+    remove_column :topic_allowed_groups, :integer if column_exists?(:topic_allowed_groups, :integer)
   end
 end