update slack hook to use new format

Updating slack incoming webhook format to match their format for incoming webhooks (which is a unique URL with token - `https://hooks.slack.com/services/<token1>/<token2>`). You just need to use this webhook url directly.

Note - this is a breaking change. If you have an existing slack hook in the old format, you'll need to delete it and it back with the new format.

The prior format seems to be supported, but unsure for how long.