Add processor filter to webhooks list page

Implements #14 (closed)

• Provides a drop-down to filter the received webhooks by the processor.
• All processors are listed, with their type, name, live/test and a summary count of webhooks received.
• (screencast below also includes an ID number before the type, name, but this was just for testing and is not included in the MR)

merged

Thanks @artfulrobot

Make sure to run "Update Caches" after applying this patch. I know this should be done anyway and always but our provider must have missed this step, resulting in a loss of Webhook functionality.