Making sender name go in-line with message body only if
the html starts with <p> tag since it won't look good
if the message starts with a code snippet, ul, etc.
If message starts with p tag we can safely assume that
it can go in-line with sender name.
As part of this change, we port into the .messages class the work in
4e8e7348da to change overflow-y to auto,
not scroll (skipping that would result in a regression).