[dw-free] Need to record previous expiration of a now-seed-account

[fu]

[commit: http://hg.dwscoalition.org/dw-free/rev/f86159d4bcfe]

http://bugs.dwscoalition.org/show_bug.cgi?id=3647

Don't destroy the previous expiration date, so we can see how much paid time
is transferrable.

Patch by fu.

Files modified:

• cgi-bin/DW/Pay.pm
• htdocs/admin/pay/index.bml
• t/paid-time.t

( Diff: 3 files changed. )