Enable donut option for Highcharts and Google Charts (there's still work that needs to be done to these for multiple Chart attachments). Also adds a message to the Charts API Example module if the Charts default settings have not been configured.

Fixes: 2879174 and 2879294. Also improves a bug to C3 and Google that made it so that attachments were displaying the wrong chart type.

This commit takes all sub-module-specific code out of the core charts module, making the module more extensible. It also improves the libraries.yml files and removes unused code from various parts of the site.