admin: Migrate from Highcharts to C3

This patchset migrates Admin's charts section from [Highcharts](http://www.highcharts.com/) to [C3](http://c3js.org/), due to the stricter licensing model of Highcharts. Any references to the `ADMIN_ENABLE_CHARTS` setting that was used to disable the charts has been removed from the docs and the code.