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

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".

```diff
- 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.