Back-office email receipt box disappears for auto-renew

On the back-office membership for the 'send_receipt' checkbox disappears when an membership with compulsory auto-renewal is selected.

Note that is is appropriate for the message text box to not be shown in this case (as we have no way of storing this message) but the check box should exist and should be stored into the contribution_recur table. The message should make it clear that email receipts will go out each time a contribution comes in if this is set and that the message would come from the domain sender (as we don't have storage for from information)