[SLACK] color side bar is not working in notification messages

[wreulicke]

Currently (1.15.0 or higher), the color sidebar is not working.
The "color" is only available for an element in attachments.
But, now, "blocks" is placed outside of "attachments".

So, to get the side color bar, we should change the position of the "blocks".

-	return &slack.WebhookMessage{Attachments: []slack.Attachment{{Color: clr}}, Blocks: &blocks}, nil
+	return &slack.WebhookMessage{Attachments: []slack.Attachment{{Color: clr, Blocks: &blocks}}}, nil

But, the attachments field is not recommended.
ref: https://api.slack.com/reference/messaging/payload

I cannot find a way to add a color side bar in Block Kit.

[wreulicke]

Migration guide for Block Kit says color parameter does not have an alternative in Block Kit.
https://api.slack.com/messaging/attachments-to-blocks#direct_equivalents