First attempt to add highcharts to graphing backends.

This backend supports line, area, stacked area and spline.
(Should work with other types, but I don't think that's revelant for graphite usage)
Highcharts seems to crash when there is a large amount of data in area mode.
Add support for highcharts-more and highcharts exporting.
Implement chart onclick redirect to inspect page of graph-explorer

Documentation is NOT updated by this pull request.