due dates, dues info mails, overdue

[Jumpy-Squirrel]

• due date handling edge cases:
  • due date should be calculated from the first unpaid or partially paid dues payment (even if there is a compensating dues transaction later). Otherwise adding and removing sponsor while approved causes the due date to move
  • due date should never move back in time (to allow manual overrides, and to avoid unfair situations)
• while in status approved / partially paid (and only then), any changes in total dues, current dues, or due date should trigger an info mail, even if no status change is involved. Right now it is hard for attendees to realize when their dues have changed since the last mail
• overdue search (if specified at all, both Yes and No) should imply an extra condition status in ( approved, partially paid ), because the question "is the attendee overdue?" only makes sense while they have one of these status values