Properly validate site ids in SpecialSetSiteLink

Using SiteLinkTargetProvider which the api also uses.

To prevent things like:
https://www.wikidata.org/w/index.php?diff=181731621

Change-Id: Iec23f779382dc8b719d8c82d89377f4663801255
Merge "Properly validate site ids in SpecialSetSiteLink"