UI - Expiry date behaviour

[ralovely]

Currently, the expiry date is a mandatory field, with a placeholder but no default.

• It should have a default.
  Default should be, in this order a new “default expiry” setting.
  This new setting should itself default to 8 days.
  if max expiry < default expiry, log a warning and set both to the lower value
• The calendar, shouldn’t allow a date beyond the limits (before today - max expiry if defined)