WIP: Migrate Namespace#root_id column

What does this MR do?

Prework of https://gitlab.com/gitlab-org/gitlab-ce/issues/62214

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

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers

Backend to do

• 1. Add a new column on Namespaces: root_id
  • This new column should be NOT NULL, but we can't enforce that limit until we have migrated the data.
  • This new column will store the root namespace equal to the namespace ID, so the child namespaces will use the ID of the root namespace.
• 2. Create a background migration to fill in Namespaces root_id
  • This migration would iterate over root namespaces only (parent_id = NULL), then run the usual CTE code to find all child nodes, then update all those.
• 3. Include logic to fill root_id when a namespace is created/updated

Next iteration

• Once the background migration has been completed, add another migration to include an index on root_id and to make it not null