Add more Slack notifications for issue and merge request events

The Edit Services page now includes settings for triggering issue and merge request events. Note that only the Slack service supports these messages at the moment.

Before:

After:

I added an execute_services() call in the base_services.rb of both issues and merge events. Git push data objects are now distinguishable from issue/merge hook data via the object_kind entry. Please check if that is the right thing to do.

This addresses #595 (closed).