Slack block message formatting

[Aketzu]

Slack has a few issues with message formatting,

With 0.20 previous text + legacy attachment was changed to block kit but this creates a fancy side effect:

Desktop Slack adds max-width: 600px to all block content which makes it look rather stupid. On the other hand when using CallbackID-attachment the same desktop Slack client renders empty attachment listing which adds some extra space between messages.

On mobile both ways look pretty much identical

My preference would be to change back to previous or make this an option for the user.