Slack: add an option to send the image as a new message

[jgournet]

When slack is configured via bot token, it can only send images as part of the thread of the alert message. Would it be possible to add more options ? Whatever is easiest, between:

• same as old notification where the screen shot is a separate message that comes right after the alert message.
• or as part of the message itself (either via templating or via an option).

[JacobsonMT]

Hi @jgournet, I'll add this as a feature request in the backlog but it is not currently on our roadmap. Feel free to open a PR.