[Bugfix] Confirmed / Unconfirmed events over time for conference dashboard

This is a bugfix for the confirmed / unconfirmed events over time chart on the conference instance dashboard!

To get the old information it was necessary to write a migration that iterates through all versions of events and increments / decrements the states!

The states getting now saved each week to the events_per_week column of events.

@hennevogel I remove the right padding from the other charts in a separate PR.
@kalabiyau @hennevogel What is with this suggestion of HoundCI? Do we really want to use next instead of each?