Nbconvert update and fixes

[david-yz-liu]

Motivation and Context

This fixes the displaying of Jupyter notebook files in the grader interface and file preview. There were two issues I encountered:

1. Running nbconvert raised this error. Updating nbconvert to 6.5.0 fixed this.
2. PR Added The Ability to Upload URLs #5822 introduced the frame_src CSP, which began preventing Jupyter notebooks from rendering in iframes. Adding :self to the allowed frame_src list fixed this.

Your Changes

Description: Updated nbconvert dependency to 6.5.0 and added :self to frame_src CSP.

Type of change (select all that apply):

• Bug fix (non-breaking change which fixes an issue)
• Other (please specify): dependency update

Testing

Verify that Jupyter notebooks correctly render in the browser.

Questions and Comments (if applicable)

Checklist

• I have performed a self-review of my own code.
• I have verified that the pre-commit.ci checks have passed.
• I have verified that the CI tests have passed.
• I have reviewed the test coverage changes reported on Coveralls.