Refactor Hashed Storage for future Rollback functionality

What does this MR do?

• Removes the "(Experimental)" text from the settings page
• Refactors Hashed Storage migration in preparation to implement a Rollback functionality in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/23955
• EE Backport in: gitlab-org/gitlab-ee!9175
• Move storage migrator workers to its own queue namespace :hashed_storage as well as move to HashedStorage folder/module

What are the relevant issue numbers?

• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/23955
• #53966 (closed)

Does this MR meet the acceptance criteria?

• 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