Assignment: Send student submission receipts should be configurable per-instance

Assignment has a site-level config setting sendsubmissionreceipts which sends email notifications to users when their submission is editing.

There are situations where this notification is not only unnecessary but confusing to students - Assignment is used for many applications, and not all need students to be aware of a submission. If the assignment is not visible to them, they are notified about a submission to an assignment they didn't know existed!

This setting should moved to a per-instance setting on assignment, with a site-level setting to lock the per-instance setting to a given value to ensure compatibility with existing installations.