Make wbclient.linkItem more user friendly

Changes:
1. The item the page links goes from is now linked when conforming
   page links
2. A meaningful error is shown at an early stage if the page the
   user wants to link to is already linked with an item having a link
   to a local page
3. Some minor improvements and removals of old temporary code

This breaks backwards compatiblity on the client but that doesn't
matter as all that isn't used in production anywhere yet.

Change-Id: I60278018d749d3bba0debb201d2df23fa9e5b9bc
Merge "Make wbclient.linkItem more user friendly"