make accept until date 1 day later than end date by default

Make accept until date 1 day later than due date by default,

The rationale is that if there's some kind of network outage, then a 10 minute grace period is very short. We've changed the default grace period to one day after being stung by such an outage. It can easily be modified but a lot of staff just accept the default suggestion.

The open date, due date, accept until and peer evaluation dates are now configurable.

Also make the assignment visible date configurable.