Add support for slack webhook notifications (just like Discord)

[jabelone]

Is your feature request related to a problem? Please describe.
The Discord integration works really well for logging swipes and issue cards but not everyone uses Discord and some spaces run on Slack.

Describe the solution you'd like
This should work in exactly the same way as the existing Discord integration and trigger whenever it does. We just need to check if Slack is enabled and post there instead of (or in addition to if both are configured). We need to make sure that each type of integration like this is handled properly if any were to fail to execute, so that any other ones work (ie if discord fails then it should skip and continue onto slack if enabled, etc.).

Describe alternatives you've considered
There's not really any alternatives.

Additional context
Will be useful for @plex3r