Add a unique index on project_id to the Geo project_registry table

What does this MR do?

Add a unique index on project_id to the Geo project_registry table and a uniqueness validation at Rails level.

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

No.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• 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 it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #3354 (closed)