jQuery 3.2.x minor security vuln - update to 3.4.x or patch 3.2.x

There is a minor and relatively hard-to-exploit security vulnerability in jQuery 3.2.x that ships with currently security-supported versions of Moodle e.g. 3.5.x.

More information on the vulnerability patched in jQuery 3.4.0:

• https://blog.jquery.com/2019/04/10/jquery-3-4-0-released/
• https://www.zdnet.com/article/popular-jquery-javascript-library-impacted-by-prototype-pollution-flaw/

Two options I see that would fix this:

1. Update to jQuery 3.4.x and backport the other fixes from MDL-65751; or
2. Patch the jQuery 3.2.x version as mentioned on the jQuery release page
   https://blog.jquery.com/2019/04/10/jquery-3-4-0-released/
   The latter looks pretty easy - a one-line patch.

Ref MDL-65751