As admin, I want to filter submission statistics based on their timestamp

[joeribekker]

Followup on #4930

To reflect the number of submissions over a period of time, we need to change the statistics page.

Tasks

1. Remove the existing date filters and timeline hierarchy from the statistics admin page.
2. Replace the counters with an aggregated count of the timeline logger events
3. Make sure the date/time filter affects the counts as expected