Pass token authorization for SSE using MS implementation

[krassowski]

Reference Issues or PRs

Uses @microsoft/fetch-event-source implementation of event streams which allows to pass additional headers. It also uses page visibility API to close the connections when not needed, reducing the server load.

This allows to pass the token along which is required to authorize with JupyterHub when running on a different server than the main one.

What does this implement/fix?

Put a x in the boxes that apply

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds a feature)
• Breaking change (fix or feature that would cause existing features not to work as expected)
• Documentation Update
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no API changes)
• Build related changes
• Other (please describe):

Testing

• Did you test the pull request locally?
• Did you add new tests?

[github-actions]

👈 Launch a Binder on branch nebari-dev/jupyterlab-gallery/sse-with-headers