notification/slack: update existing messages with status

[mastercactapus]

Description:
This PR updates the Slack notifier to include a color bar with alert messages and update the existing message instead of posting status updates in-thread.

Unacked alerts will have a red bar and details included:

Acked alerts will reflect the latest log entry, a yellow bar, and retain details:

When escalated, the alert summary and link is broadcast to the channel:

When closed, details of the alert is hidden, bar turns green, and the link remains with the last log entry:

[Forfold]

As a personal preference, I think it could look a bit cleaner without the nested quote line for the details- LGTM!

[mastercactapus]

@Forfold Maybe we can try tweaking it during the two-way interaction work; we'll be changing the rendering logic for that into its final state. I added it so it would stand out since there's no pre-processing for markdown -> slack markdown, but I agree the double color-bar isn't ideal.

[Forfold]

@mastercactapus That sounds good! Yeah I definitely don't think something like that should hold us back, so it has my approval :)

[Forfold]

It could be neat to have a button to show the full details (rendered with mkdwn) as a modal within Slack

[dctalbot]

Code looks good, but seeing some odd behavior...

I created an alert with a summary of "test", and got the following -

Native notification on mac is missing preview:

There is a stray greater than symbol in the message body:

It only goes away if I close the alert.