On registration, accept all invitations to user's email instead of deleting

The current 5.x-2.x-dev code will handle multiple invitations to a user by deleting all but the one that was accepted. It then sets up invitation notifications for all inviting users, but all except the first have already been deleted. For the deleted invitations, the user who sent the invitation loses it without a trace.

This patch will accept all of the invitations, which will fix the notification problem as a side-effect.

I think this is the right behavior, at least for my site. All users who sent an invitation to this user will see it show up as a successful invite.