Update diff2html and necessary scripts

[parlough]

See main differences in site-shared PR (dart-lang/site-shared#163).

• Update to latest compatible versions of diff2html and highlightjs without larger changes
• Add integrity checking to the scripts
• Remove jquery usage with modern (and faster) built-in methods
• Fixes error caused by jquery not being present yet sometimes

After:

Before:

Issues fixed by this PR (if any): Fixes N/A

Presubmit checklist

• This PR doesn’t contain automatically generated corrections (Grammarly or similar).
• This PR follows the Google Developer Documentation Style Guidelines — for example, it doesn’t use i.e. or e.g., and it avoids I and we (first person).
• This PR uses semantic line breaks of 80 characters or fewer.