Upgrade to jQuery v3

Upgrade to jQuery v3

Source code:
 https://code.jquery.com/jquery-3.2.1.js
 https://code.jquery.com/jquery-migrate-3.0.0.js

Documentation:
 https://blog.jquery.com/2016/06/09/jquery-3-0-final-released/
 https://jquery.com/upgrade-guide/3.0/

This is not a breaking change because jQuery Migrate covers
all breaking changes.

However some extensions (especially unit tests) may've relied
on undocumented behaviour. For that reason, and due to unresolved
upsteam issues this is still behind a feature flag for now.

It is true by default to ensure this has wide exposure to discover
issues as quickly as possible. If this is not resolved before
the end of the 1.29 release cycle it should be turned off again.

Bug: T124742
Change-Id: I3c3dedaa9a9d449eaa2b7e5d24b4540e7fa421c0