Add Application Setting to configure default Repository Path for new projects

What does this MR do?

Add a new Application Setting to configure default Repository Path for new projects

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

Is the UI ok?

Why was this MR needed?

Give an interface to !4578 (merged)

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/issues/583

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together

/cc @pcarranza