admin: Migrate from Highcharts to C3

For every chart that uses Highcharts, add its C3 counterpart below it.
The change from Highcharts to C3 is for license reasons.
admin: Lift restriction on charts

Remove `ADMIN_ENABLE_CHARTS`, as we now use C3 instead of Highcharts.
docs: Remove Admin charts section

Due to the migration from Highcharts to C3, the charts are now enabled
by default.
Merge branch 'release-0.16-admin-charts' of https://github.com/apyrgio/synnefo into release-0.16