[Assignments] Enable students to delete their assignment submission

[KyraK]

Sometimes a student accidentally uploads the wrong file. Instead of resubmitting a new file, and thus having submitted two files, it would be better to be able to delete your submission!

[dspiel]

This seems like a no-brainer and should be created.

[dspiel]

Having this feature would be extremely helpful if it was where only instructors could remove a submission, but there could also be a setting similar to quizzes where you get an extra attempt for just one student, maybe had something within assignments where you could tag just a particular student to submit an additional upload...