Add middleware to prefix JupyterHub navbar items with /hub.

[marcelovilla]

Reference Issues or PRs

Closes #2359

What does this implement/fix?

Put a x in the boxes that apply

• [x] 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

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

Any other comments?