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

lib/mini_sql_multisite_connection.rb

@@ -36,6 +36,9 @@ class MiniSqlMultisiteConnection < MiniSql::Postgres::Connection
 
     def before_committed!(*); end
     def rolledback!(*); end
+    def trigger_transactional_callbacks?
+      true
+    end
   end
 
   # Allows running arbitrary code after the current transaction has been committed.