Charts API: support for legend options for Chart.js

This patch branch provides a means to set legend options as per: https://www.chartjs.org/docs/2.7.0/configuration/legend.html

It adds a field, setters and getters to the Base chart classes in PHP and Javascript. From Moodle, it is invoked via e.g.: $chart->set_legend_options(array('position' => 'bottom', 'reverse' => true))

At this point, specifying onClick and onHover events is not directly supported via this method. Dynamically creating functions out of strings is not permitted by the ESLint no-new-func directive, and it's arguable as to whether that would be a good approach to take in the first place.