[UI] Jupyter notebook export patch

[senwang86]

Summary

• For Jupyter notebook export
  • Better align the pod result with Jupyter cell output format, issue Codepod-exported ipynb not compliant to NBFormat 4.0 #411
    • Add error output
    • separate execute_result and display_data
• For Jupyter notebook import
  • Support error imports
• Improve exported .ipynb readability
• Add repo_id into the top-level metadata

[lihebi]

Thanks!