Allow 'auto' legend layout in Highcharts.js

Problem/Motivation

The Chart configuration > Highcharts Settings, allows configuring the Legend Layout to Horizontal or Vertical. This layout will be applied to the legend regardless of the position of the legend (controlled from within each chart).
This can result in bottom legends with vertical layouts, or side legends with horizontal layouts; in either case the legend takes up a lot of valuable screen space.

Steps to reproduce

Proposed resolution

It appears the code previously allowed chose the legend layout based on the position of the legend as long as there was no "layout" value assigned globally. However, the current configuration of Highcharts Settings forces one to select a value. If we allow this field to be blank or create an 'Auto' option as blank, then the legend layout will be selected automatically allowing charts to use the best layout based on the position of the legend.

Remaining tasks

User interface changes

API changes

Data model changes