WIP: Adds deleted-in-version to designs

What does this MR do?

This adds a column, deleted_in_version_id to the DesignManagement::Design model, marking them as deleted.

This is in support of https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/14656

Database checklist

• Conforms to the database guides

When adding migrations:

• Updated db/schema.rb
• Added a down method so the migration can be reverted
• Added the output of the migration(s) to the MR body

$ rails db:migrate
== 20190711111748 AddDeletedInVersionToDesignManagementVersion: migrating =====
-- change_table(:design_management_designs)
   -> 0.0270s
== 20190711111748 AddDeletedInVersionToDesignManagementVersion: migrated (0.0271s)

• Added tests for the migration in spec/migrations if necessary (e.g. when migrating data)

General checklist

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides