Allow teachers to give extended deadlines for individual students

This could be achieved by creating
GracePerioid{course, exercise, student} and then GracePerioid <-->Submission relation.

This would make it possible for individual students to start a hard deadlined course after the first deadline has passed.

Changewise this wouldn't change much and should be pretty straight forward to implement.

It would add a minor cost for calculating deadlines for the json and ui; though we already have the DeadlineSpecObj this shouldn't cause much of additional expenses..

As well for SubmissionsController it would be easyish to check whether grace perioid is being used.
And submission could just show that it was submitted on grace period.

For points, I'd consider this to modify only the hard deadline and not to have any effect on soft deadline. (And for ui/json to have a flag for teachers to see that the grace period was used.)

Start by reading the existing deadline calculation around DeadlineSpecObj, the Submission model, and SubmissionsController. Define how an individual grace period changes only hard deadlines, how JSON and the UI expose its use, and how teachers can identify grace-period submissions; done means these behaviors are covered and existing soft-deadline behavior remains unchanged.