Geo: Implement project wiki repository replication for organization migration

Problem

As part of organization migration from Legacy Cell to Protocell, Geo must replicate project wiki Git repositories. But repository storages differ between cells, and repository storages are managed in the PG DB. Therefore PG replication requires some kind of special handling of repository storages. By chance, since project wikis repos reuse the same storage as project repos, in this issue, the main task is testing.

Background

• Project::WikiRepository delegates #repository_storage to the associated Project.
• project_wiki_repositories is already sharded by project_id
• This issue is blocked by Geo: Implement project repository replication f... (#577735) because we will rely on projects.repository_storage being properly updated as part of replicating projects data.
• On migration, assuming the projects.repository_storage process above is working, Geo can sync project wiki Git repos as normal.

Proposal

• Classify this table as gitlab_main_org (it is currently gitlab_main_cell)
• Ensure there are sufficient automated (unit? QA?) tests to meet GitLab's standards
• Smoke test locally

Related

• Parent epic: Organization migration: Handle replicables of c... (&18601)
• Architecture decisions: #515383 (comment 2743597917)+
• At the moment, depends on: Geo: Implement project repository replication f... (#577735)