(bug 43870) new Special:SetSiteLink

New special page for setting a site link of an item. New class
SpecialSetSiteLink created. New baseclass SpecialModifyEntity created.
SpecialSetEntity and SpecialSetSiteLink now inherit from SpecialModifyEntity.

Change-Id: I0c6c8432f4d792f9b0a011d8434ad2ac52333e9f
Merge "(bug 43870) new Special:SetSiteLink"