Convert lektor_service_name in a generic envvars_localized plugin

With a few tweaks, the lektor_service_name plugin can be converted into a in a generic envvars_localized plugin offering an unified way to translate content that's configured via environment variables.

A envvars_localized('VARIABLE_NAME', this.alt) could be used anywhere in the templates to get the localized value for a given variable, or the default.

This function would make the following lookups:

1. Try to resolve a service name via LEKTOR_VARIABLE_NAME_${LANG_CODE} environment variable.

2. If the variable above is unset, the function fallback to the value of the LEKTOR_VARIABLE environment variable, or an empty string ('').