Add deletion support for designs

What does this MR do?

This adds the capability to destroy/hide designs. See the related issue

This is an edited and 'reduced' version of https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/14046. I do not think it will be possible to reduce this in size much at all (in fact I believe it needs more tests).

This is the backend component. The frontend work is to be found in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/15034

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

This poses no significant performance risks. It only includes the backend component, so there is no browser testing needed.

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.)

/cc @phikai @.luke

Part of the work for: https://gitlab.com/gitlab-org/gitlab-ee/issues/11089