Add a parameter setting for space inside a message group

Hey. There are some "unfortunate" changes to the new discord UI, but I actually think the customization possibility for spacing between message group is a really great addition.

However, I think what would help a lot as well is to allow such a spacing option for messages that are part of the same group. That way, people can adjust the visual to be more consistent if they want it, and make it a bit more calm.