Remove invite after user successfully registers
Remove invite after user successfully registers

An invite code can only be used once, so we should remove it once we've
successfully signed the user up and given them permission to access the
instance.

mysociety/popit,Sinar/popit,Sinar/popit,Sinar/popit,openstate/popit,openstate/popit,mysociety/popit,mysociety/popit,mysociety/popit,openstate/popit,mysociety/popit,openstate/popit,Sinar/popit