Allow end dates of auto-renew memberships to be edited

It is convenient to be able to adjust the expiration date for auto-renew memberships, even though the auto-renew system will override that date upon the next billing cycle. A example use-case for this is when there's a temporary failure within the system that results in a successful PayPal recurring payment, but Civi doesn't process it. With this patch, back office staff can record the contribution manually (as they already can), adjust the membership's expiration date, and let things continue as normal during the next automatic billing cycle.