Translatable URLs in the translations repository

Make the following changes at the Onion Launchpad translation:

1. Replace envvars('SERVICE_URL') by envvars_localized('SERVICE_URL', this.alt).
2. Replace envvars('ONION_URL') by envvars_localized('ONION_URL', this.alt).

This allows having the same landing page offering different URLs depending on the selected language.