Assignment: model

Add the following properties to Assignment entity:

Type	Name	Comment
Integer	lateSubmissionLeniency	How late a submission can be (hours) to still be accepted
Boolean	acceptLateSubmissions	Whether late submissions should be accepted (but marked as late)
String	approveScript	Link to approve script (this might change)
Integer	limitNumberOfSubmissions	How many submissions can a group submit
String	limitFileTypes	Limit what file types can be submitted