Using the same Task multiple times in a Contest

[maxhaslbeck]

If one uses the same Task multiple times in a Contest (e.g. fun modulo 5 in AllTime) push-tasks can not identify which task (combo of task_definition, contest, and name) is meant. For updating the descriptions and task resources that is no problem. But when additionally wanting to submit a solution, push tasks needs a contest to submit to.

At the moment one has to drop the sample solution (Submission.thy for Isabelle).