Update backbone.js to 1.4.0

Problem/Motivation

As part of #3094468: [plan] Update core JavaScript (and CSS) dependencies prior to 9.0.0-beta1 we need to update backbone.js
Version 1.40 is available, released Feb 19, 2019 *changelog*.

Proposed resolution

Update it

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet