Migrate spec/javascripts/notebook/ to Jest

Description

Let's migrate the specs in spec/javascripts/notebook/ from Karma to Jest.

See the related epic for how to do this.

• spec/javascripts/notebook/cells/markdown_spec.js !31429 (merged)
• spec/javascripts/notebook/cells/prompt_spec.js !31429 (merged)
• spec/javascripts/notebook/cells/output/html_spec.js !31429 (merged)
• spec/javascripts/notebook/cells/output/index_spec.js !31429 (merged)
• spec/javascripts/notebook/cells/output/html_sanitize_tests.js !31429 (merged)
• spec/javascripts/notebook/cells/code_spec.js !31429 (merged)
• spec/javascripts/notebook/index_spec.js !31429 (merged)