gdash: add stub-quality VisualEditor dashboard

Add a dashboard to Gdash with ve.performance.user.saveComplete and
ve.performance.system.activation Q2 and Q3 values for a fixed one-minute
sliding window. This does not amount to an adequate view of VisualEditor
performance (we should probably have a histogram or a flame chart for timing
data so we can keep an eye on performance in the top quantile/decile, and we
should look at weekly / monthly perf data), but it provides sample code
that the VisualEditor team can extend.

Change-Id: I6352f1e9f278432b08a9771579bd58e65d514738