Allow quiz deletions

[danielemery]

Allow a user to delete a quiz, when:

• They uploaded it
• No one has recorded a completion against it yet

Note this means the error case should be explicitly handled when:

1. Someone uploads a quiz
2. A group starts completing the quiz
3. The uploader deletes the quiz
4. The group wants to record their results

One possibility is to leave the deleted quiz there but in a clearly marked pending deletion state and allow the deleter to provide a comment like "reuploaded with the correct date".

For now soft deletion is enough.