Fix Dependabot jQuery XSS potential vulnerabilities

[DrDrij]

https://github.com/QuantEcon/website/security/dependabot

Dependabot has been updated to warn for old versions of jQuery.

Check if jQuery is needed or update to a newer version and check all functionality.

[mmcky]

Dependabots is now showing no issues.

@DrDrij can we configure dependabots to do suggested updates?