WIP: Add IPython notebook preview

What does this MR do?

Adds support for previewing IPython notebooks using notebookjs library. IPython notebook is detected using extension .ipynb. If such file is opened in repository the notebook preview is rendered instead of plain JSON. You can switch back and forth between the rendered preview and the source.

Why was this MR needed?

To address #2508 (closed)

Screenshots (if relevant)

Valid notebook preview

Invalid notebook

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 it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #2508 (closed)