(bug 51855) remove code that assumes site links from only one site group

This makes the change comments for site link changes generic,
and stores the site id instead of "lang".

This allows things work with multiple site groups (e.g. wikivoyage),
and allows in the future, more elegant comments for site link changes.

Change-Id: I071e9dad970144e12aefa1296a8cb4f057d36ab9