Make possible to click directly on burndown chart to go to each test plan

[Ursula Junque]

noodles suggested to add a link to each test plan in the burndown chart [1], because that would be more helpful than using the PreReleaseQAReport page [2].

[1] http://people.canonical.com/~lpqateam/test-plan-report-3.1.10.html
[2] https://wiki.canonical.com/Launchpad/QATeam/PreReleaseQAReport/3.1.10

[Diogo Matsubara (matsubara)]

BarChart and ColumnChart objects from Google's visualization API doesn't support allowHtml option, which is required to be able to properly insert links into the chart. See http://code.google.com/apis/visualization/documentation/gallery/barchart.html#Configuration_Options for the options.

See diff on https://pastebin.canonical.com/28332/ for what the code would look like if the BarChart accepted the allowHtml option.