upgrade jquery and grunt to more recent versions

[gyulalaszlo]

Fix Dependabot issues with JQuery and Grunt -- none of the mentioned specific problems are relevant for this specific code, but the versions used in the repo are generally very-very old, and should be updated if possible:

JQuery:

Passing HTML from untrusted sources - even after sanitizing it - to one of jQuery's DOM manipulation methods (i.e. .html(), .append(), and others) may execute untrusted code.

Grunt:

The package grunt before 1.3.0 are vulnerable to Arbitrary Code Execution due to the default usage of the function load() instead of its secure replacement safeLoad() of the package js-yaml inside grunt.file.readYAML.

I've done some minimal testing and can confirm that it builds and the basic pages load without problems (looking at the JQuery major changelog for 3.0 I dont think there are any breaking changes, but my assumptions are not good replacements for real-world tests.