linebreaks are a little wrong in multi-line messages vs. grouped messages

[holmesworcester]

Right now linebreaks are a little wrong in a way that impacts readability. The linebreaks within a message are bigger than the linebreaks between multiple grouped messages. They should be the same, and we should copy Slack for the linebreak height.

I think it should be something like:

• Newline within message: 1 line, same as normal text wrapping line height.
• Double linebreak within message: 1.5 lines
• Linebreak for grouped messages: 1.5 lines

We should look at what Slack does.