Allow to configure default settings for JupyterLab (overrides.json)

[krassowski]

Reference Issues or PRs

Closes #2247

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: Document default_settings for jupyterlab nebari-docs#407
• 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?
  No, I could not setup nebari locally due to Error: client rate limiter Wait returned an error: context deadline exceeded error
• Did you add new tests?
  Yes, I added a new config yaml file for happy path verifying that jupyterlab.default_settings does not error out, and an end-to-end test

Any other comments?

[aktech]

Looks pretty good to me! 🎉