[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.
I guess what @houndci is trying to tell you is to skip the iteration with: 

```ruby
next if blah 
```
instead of 
```ruby
if blah
end
```