New: renew an expired subscription from its page

When an expired subscriber wants to come back, you can now renew the subscription from its page instead of rebuilding it by hand.

An expired subscription now shows a Renew action. It opens Create subscription filled in from the expired one, with a note saying which subscription it came from. Any product that is no longer for sale is left out, and the form tells you which.

A subscription created with Renew also carries the plan's payment limit, when the plan has one, so a renewal stops at the plan's limit like any other. Before this change, a subscription rebuilt by hand did not take over the plan's number of payments and never expired.

Both Create subscription forms now draw "Charge first payment immediately" as a switch. On both forms, Save needs either a first payment date or the switch turned on. On a renewal the date starts blank, so you choose one or turn the switch on before you can save.