Add cookie path for CentralAuth cookies

Just as core has $wgCookiePath, this allows admins to set a cookie path
for CentralAuth cookies. This allows for multiple wikis to be in
different paths under the same domain, all connected with CentralAuth.

Bug: 70891
Change-Id: Ic10d72c2e0df4cc3f1360fe83b1a644dfe77f6ab
Merge "Add cookie path for CentralAuth cookies"