fix computation of weekly/monthly pct in attention chart

[rahulbot]

Right now, if you view an Explorer chart by week or month, and switch to a normalized percentage view, the results are a little off. They are showing you the average percentage over that time period, rather than the true percentage. Those are not the same :-(

This is because the data to aggregate wasn't available to the low-level attention chart, which does the aggregation. To fix this, we have to move that aggregation logic up, unfortunately. This will be a less-clean and less-generalizable architecture, but will be more accurate.