Assignments: Fill in dates by default, when we "Add" more students to "Assign to"

[tomasdore]

Request: when we "Add" more students to "Assign to" for an assignment, the dates should copy by default, rather than being blank. 

It would prevent this happening:

1. OK, we've set up an assignment for an exam but we've been careful not to release it just yet - we set the "Available from" date. Good! It's essential they don't see the exam in advance!
2. Oh, but some students have a requirement for extra time? Yes, OK, we go back in, click Add, enter their names, save it, all done, right?
3. A few hours later.... an email from one of those students who gets extra time. They can see the exam?! Why?

_What happened?_

You worked it out? The dates are not copied by default. Now "Available from" is empty for the students we added. So they can see the exam. 

This is strange, as this date typically does not change: it's the "Due" and "Until" dates that we would typically want to vary. 

[ProductPanda]

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