Fix deprecation warnings around class loading during initialization

```
Fix deprecation warnings around class loading during initialization
Thi2022-11-08T08:54:57.542Z pid=40093 tid=nl9 WARN: DEPRECATION WARNING: Initialization autoloaded the constants ReportsHelper, DateTimeStringValidator et IntegerArrayValidator.

Being able to do this is deprecated. Autoloading during initialization is going
to be an error condition in future versions of Rails.

Reloading does not reboot the application, and therefore code executed during
initialization does not run again. So, if you reload ReportsHelper, for example,
the expected changes won't be reflected in that stale Module object.

These autoloaded constants have been unloaded.

In order to autoload safely at boot time, please wrap your code in a reloader
callback this way:

    Rails.application.reloader.to_prepare do
      # Autoload classes and modules needed at boot time here.
    end

That block runs when the application boots, and every time there is a reload.
For historical reasons, it may run twice, so it has to be idempotent.

Check the "Autoloading and Reloading Constants" guide to learn more about how
Rails autoloads and reloads.
```

config/application.rb

@@ -155,15 +155,17 @@ module Openfoodnetwork
 
     initializer "ofn.reports" do |app|
       module ::Reporting; end
-      loader = Zeitwerk::Loader.new
-      loader.push_dir("#{Rails.root}/lib/reporting", namespace: ::Reporting)
-      loader.enable_reloading
-      loader.setup
-      loader.eager_load
+      Rails.application.reloader.to_prepare do
+        loader = Zeitwerk::Loader.new
+        loader.push_dir("#{Rails.root}/lib/reporting", namespace: ::Reporting)
+        loader.enable_reloading
+        loader.setup
+        loader.eager_load
 
-      if Rails.env.development?
-        require 'listen'
-        Listen.to("lib/reporting") { loader.reload }.start
+        if Rails.env.development?
+          require 'listen'
+          Listen.to("lib/reporting") { loader.reload }.start
+        end
       end
     end
 
@@ -247,5 +249,7 @@ module Openfoodnetwork
     config.active_storage.variable_content_types += ["image/svg+xml"]
 
     config.exceptions_app = self.routes
+
+    config.autoloader = :zeitwerk
   end
 end