fixed EXT-3818 “[BSI] plain text chat has too much space between lines”,

To fix EXT-6373 I added vertical padding between posts in plain-text and
unified padding in default mode of IM/chat history.
Now I'm backing out the changes to plain-text mode that led to EXT-3818.
reviewed by Leyla Farazha at
https://codereview.productengine.com/secondlife/r/95/

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branch : product-engine