Allow Slack service to notify status of builds on a different channel

• Zendesk ticket: https://gitlab.zendesk.com/agent/tickets/14648

Requested feature

Now that there are no specific CI services, the Slack notifications for CI builds has been merged into the normal Slack service. This can cause unrelated messages to be posted on a channel where you don't want to have messages about builds.

Adding a setting where you can choose the channel to post to for build messages can be really useful.

Proposal

1. Remove the #Channel field
2. Add #Channel fields to each event type

Concrete questions / Next steps

@JobV what do you think?