[slack] allow the user to choose the message format to send

[DaanMeijer]

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Description

Fixes #3226

Type of change

Please delete any options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• User interface (UI)
• New feature (non-breaking change which adds functionality)
• Breaking change (a fix or feature that would cause existing functionality to not work as expected)

Checklist

• My code follows the style guidelines of this project
• I ran ESLint and other linters for modified files
• I have performed a self-review of my own code and tested it
• I have commented my code, particularly in hard-to-understand areas (including JSDoc for methods)
• My changes generates no new warnings
• My code needed automated testing. I have added them (this is optional task)

Screenshots (if any)

Non-richt text message:

Rich text message:

[CommanderStorm]

Thanks, mostly mergable.
I have noted one thing regarding translation keys below.

[DaanMeijer]

@CommanderStorm To be sure: is any more action required from me?

[CommanderStorm]

Thanks for the enhancement to the notification provider! 🎉
I have changed this to v2.0, as this is more of a feature than a bugfix.

Note

This PR is part of the v2.0 merge window => see #4500 for the bugs that need to be addressed before we can ship this release ^^

_{All changes in this PR are small and uncontroversial ⇒ merging with junior maintainer approval}

[pelligrag]

Rich messages are not functioning on version 2.0 beta. I don't receive any notifications if I enable rich messages in Slack notifications.

[DaanMeijer]

Rich messages are not functioning on version 2.0 beta. I don't receive any notifications if I enable rich messages in Slack notifications.

This is fixed in #5196 (comment)