UI: Fix statistics reporting

[smathot]

There are a number of seemingly related issues with statistics reporting in the UI:

Since December 1st, the weekly participation trend resets to 0 daily. "Most active studies" and "Most active participants" panels show incorrect data. Daily stats still work correctly.

There are also zeros and nan-values that are clearly incorrect.

[BmBaczkowski]

Statistics report seems to work fine:

I am also not able to replicate the dashboard error:

How can i see the issue?

The issue with NaN is fixed in 014e161

[smathot]

I think we'll need to test this on their actual database to reproduce the issue and see if it's fixed with 014e161.