If someone that already exists in greg follows an invite, we are kind
enough to give the new role to the existing person. This introduced a
security risk if the invitation was actually meant for someone else.

Because of this situation, we introduce a security mechanism where we
disable the invitationlink if the name of the existing person is too
different from the name used in the invitation.

Resolves: GREG-166

Copied from cerebrum, uses restricted damarau levenshtein.

Issue: GREG-68