Switch user activity chart to last 4 weeks (28 days) instead of month (30 days)

[pluehne]

The user activity chart stems from before we came up with the idea to have all sliding windows and aggregation methods consider 4-week intervals instead of 30-day intervals. The benefit of this is that weekends are evened out, which avoids accidentally misinterpreting some fluctuations.

The user activity chart query should be changed, and the data format should be adapted to make things consistent.