Hashed storage migration path

What does this MR do?

Implements function to migrate projects from Legacy storage to Hashed storage asynchronously. This MR also backports from EE the repository_read_only flag introduced as part of https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/533

Are there points in the code the reviewer needs to double check?

Check if the way the migration is being made is safe / atomic.

Why was this MR needed?

For Geo we want to have all repositories using Hashed storage.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Fixes https://gitlab.com/gitlab-org/gitlab-ee/issues/3118

cc @jarv @stanhu @dbalexandre @nick.thomas