Allow download all submissions, not just those which have been assigned

[Dave Sands]

Grading projects live we would have found it useful to be able to download all submissions regardless of grader.

[Daniel Schoepe]

This would also be useful when noticing a case of plagiarism later in the grading process and trying to check if previous solutions were also affected. If one could download all solutions, including accepted / rejected ones, this would be easier to automate.

[Evgeny Kotelnikov]

Fixed by pull request #16.