Benchmark columns for datasets and submissions.

Datasets and submissions need a "Benchmarks" column to indicate which benchmarks they are used in. This can be done in the same way it is done for tasks. See the specs of the Cometh project.

Also, whenever a submission is run on a task, the tables should be updated to reflect this (Submissions associated, tasks associated, In use…). It will not be possible to:

• edit or delete a file used by a task or a submission
• delete a task used in a competition
• delete a submission that was submitted to a competition
• publish a task not validated by a submission
• publish a submission not validated by a task It should be possible to delete multiple files at the same time.