Investigate WikiSite JSON parsing, and wikidata page namespace TODO comments

[psh]

During Java -> Kotlin migration I uncovered a couple of old //TODO comments that ought to be looked into given their age -

In NamespaceTypeAdapter

// Prior to 3210ce44, we marshaled Namespace as the name string of the enum, instead of
// the code number. This introduces a backwards-compatible check for the string value.
// TODO: remove after April 2017, when all older namespaces have been deserialized.

In WikiSiteTypeAdapter

// todo: legacy; remove reader June 2018
if (reader.peek() == JsonToken.STRING) {
    return WikiSite(Uri.parse(reader.nextString()))
}

and

// todo: legacy; remove reader June 2018
return if (languageCode == null) {
    WikiSite(domain)
} else {
    WikiSite(domain, languageCode)
}