Deprecate Slack as a distribution channel

[kitallis]

Proposal

In our chats with users, it seems like it's not clear what the difference between Slack notifications and Slack as a distribution channel is. It feels like we can get rid of Slack as a distribution channel altogether and have the Notification channel handle all the distribution responsibilities as well

What this achieves / problem(s) it solves

Slack / Email as notification channels can also solve the artifact distribution problem. We don't need to have them explicitly as distribution channels also. Doing this should make the system much simpler to understand.

Also see #447