Pull import sources from Gitlab::ImportSources

What does this MR do?

ApplicationSettings.create_from_defaults defines an array of import sources, but we should be pulling them from Gitlab::ImportSources.options since that's what they're validated against.

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

The one and only line that was changed: https://gitlab.com/ojss/gitlab-ce/commit/e672c0128a232cd25112de4e84528112ec2e59d9

Why was this MR needed?

An issue was raised #21814 (closed) about this.

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 merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #21814 (closed)