Update state of all submissions when done

When a user had one or more submissions whose state was not
`superseded` and that user decided to close that exercise. Then the
latest submissions state would correctly get changed to `done`, whereas
all the old submisssions would not have their states updated to
`superseded`. This has now been fixed and updated to correctly change
the state of old submissions.

* Update completion to change state of old submissions
* Update completion_test to assert that old submissions states are updated
* Remove unused variable in completion_test