Adds a new webhook integration for Slack to receive messages
from one's Slack team's public channels.
Contains negative tests for broken, missing or invalid data.
Allows two different option for integration:
1. Receive notification on a single stream with different topics
for each of Slack's public channels.
2. Receive notification on different streams for each of Slack's
public channels.
Steps to choose between the two options is described in the documentation.
Fixes#3569.
