Validate DesignManagement::Action#event

Summary

We should validate that DesignManagement::Action records can only have one creation and one deletion event per design.

It came up in !17322 (comment 239415786) that we had a fixture for a spec that would create designs with multiple creation events. The spec passes because we don't have any validation on this.

I feel like it would be worth adding this validation, although it adds an extra database overhead to the creation of these records.