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

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).

No files or tests are named. Start by locating the Slack bot-token notification path that currently sends screenshots in the alert thread, then inspect how the prior separate-message behavior was implemented or documented. Done means one clearly specified option is supported and covered by tests for screenshot placement.

Written by the indexing model from the issue text.