[Bug 857078] KPI Charts s/Highcharts/Rickshaw/

* Replace almost all the KPI charts with Rickshaw versions. The exception
  being the questions graph, because it relies on multiple y-axises, which
  Rickshaw can't do yet.
* In kpi/base.html, make scripts and styles overridable.
* Convert kpi.dashboard.css to kpi.dashboard.less.
* Change all places where `window.k` is initialized respect existing instances.