Free up the assignment name after deleting an assignment

Description

Previously, when deleting an assignment, it would hold that name reserved forever. That doesn't happen now. Closes #109 (closed)

Changes

• Free up the assignment name when an assignment is deleted

Test and Review

To be filled in by the reviewers

• All of the methods are commented to expectation

• The methods are tested to satisfaction

• There are no unnecessary files present in the MR

• The continuous integration has no problems with the MR

• The MR is filled in as requested (including labels, milestones, and reviewers)

• The documentation is up-to-date

• All nullable parameters are marked as such