Enable Skylight in staging

This is a follow-up of
openfoodfoundation#2070.

When running the production API key in staging, the `log/skylight.log`
showed:

```
[SKYLIGHT] [1.5.0] You are running in the staging environment but
haven't added it to config.skylight.environments, so no data will be
sent to skylight.io.
```

According to
https://www.skylight.io/support/advanced-setup#setting-up-multiple-environments
it turns out we need to create a new app for staging and use its API
key.

config/application.rb

@@ -24,6 +24,13 @@ class Application < Rails::Application
       end
     end
 
+    # Activate the Skylight agent in staging. You need to provision the
+    # SKYLIGHT_AUTHENTICATION env var in your OFN instance for this to work.
+    #
+    # Check https://github.com/openfoodfoundation/openfoodnetwork/pull/2070 for
+    # details
+    config.skylight.environments += ["staging"]
+
     # Settings dependent on locale
     #
     # We need to set this config before the promo environment gets loaded and