Slack integration is broken due to deprecated/broken Slacker

[mwakaba2]

Problem I tried setting up ntfy for slack notifications, but looks like Slacker is broken, so it's no longer able to make requests via the slack api.

I tried to make a PR with slacker to get this fix in, it looks like the project got archived and is no longer maintained.

Recommendation I think it's best to remove this slack option and recommend users to set-up slack webhooks instead.