Fill project_repositories for hashed storage projects

What does this MR do?

This is the second implementation step for https://gitlab.com/gitlab-org/gitlab-ce/issues/48527

This adds a background migration that will ensure all projects that are on hashed storage have a row in project_repositories.

With having a row in that table, it will be easy to lookup a project that is on hashed storage:

ProjectRepository.find_project('@hashed/ab/cd/acbd0000000000000000ef')

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/48527

Does this MR meet the acceptance criteria?

• 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
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer