refactor tasks & datasets

@ mention of reviewers`

@ckcollab`

A brief description of the purpose of the changes contained in this PR.

This PR adds some limitations when deleting things from the "Tasks and Datasets" page. It also allows the deletion of multiple items at once.

Issues this PR resolves

#428 (closed)

Suggested changes

I find it weird that a submission shouldn't be deletable after it's been assigned to a competition. Users aren't going to be able to delete any submissions until the competition is deleted. It makes more sense to me to disallow deletions of submissions that have been added to the leaderboard, or something along those lines.

A checklist for hand testing

• Try to delete some tasks, datasets, and files
• Make sure a file can't be deleted if it's used by a task or algorithm
• Make sure a task can't be published if it hasn't been validated
• Make sure a task can't be deleted if it's used in a competition
• Make sure a submission can't be published if it's not validated
• Make sure a submission can't be deleted if it's part of a competition
• Make sure multiple tasks and submissions can be deleted at once

Checklist

• Code review by me
• Hand tested by me
• I'm proud of my work
• Code review by reviewer
• Hand tested by reviewer Logan
• Ready to merge