[Bug 845775] Replace KB article history charts with Rickshaw

s/highcharts/rickshaw/.

To test: Go to any KB article. Go to it's history. Click "Show Helpfulness Vote Chart." Play with it.

Of note:

- Data is now grouped by week instead of by day.
- No "minimap" below the scroll bar. This isn't built into Rickshaw, and I didn't want to build a new Rickshaw component for it.
- The graph is technically a little less powerful, because of less UI elements, but I think it can do everything useful from the old version.
- I also made some of the Rickshaw stuff I wrote before a little more reusable.

Apologies for the massive diff.

r?