Upgrade the ChartJS dependency to version 2.7.3

This issue is about upgrading the extension to the currently latest version of ChartJS, that is version 2.7.3. The artefact currently used, org.webjars.bower:chartjs, is not available in version 2.73, however org.webjars:chartjs is, so the idea would be to use the latter as a dependency, unless there was a specific reason to use the former.