Cleanup AppLinks that have not been linked with a user
Cleanup AppLinks that have not been linked with a user

This provoked an error when you did:
* Start signup with OAuth
* Abandon signup
* Login with an existing account
* Try to link with OAuth.

The latter raised an error since there was an AppLink for that provider
and user account, but it was assigned to no user on Teambox.

ccarruitero/teambox,nfredrik/teambox,crewmate/crewmate,rahouldatta/teambox,vveliev/teambox,codeforeurope/samenspel,teambox/teambox,nfredrik/teambox,teambox/teambox,nfredrik/teambox,vveliev/teambox,alx/teambox,alx/teambox,rahouldatta/teambox,ccarruitero/teambox,wesbillman/projects,irfanah/teambox,codeforeurope/samenspel,vveliev/teambox,crewmate/crewmate,irfanah/teambox,crewmate/crewmate,vveliev/teambox,wesbillman/projects,rahouldatta/teambox,teambox/teambox,irfanah/teambox,irfanah/teambox,nfredrik/teambox