Add configuration options for the event queue sizes

[robbert-vdh]

As suggested in #235, it would be nice to be able to set the event queue size limits in Hoff's configuration. These were recently bumped from 10 to 128 to avoid dropping events when GitHub starts sending Hoff dozens of webhook events in rapid succession. Being able to change this limit without having to patch and redeploy Hoff would be a nice quality of life change.