Improved display of plain text messages and text to HTML conversion (#1488937)

Now instead of <pre> we use <div class="pre"> styled with monospace
font. We replace whitespace characters with non-breaking spaces where
needed. I.e. plain text is always unwrappable, until it uses format=flowed,
in such a case only flowed paragraphs are wrappable.

Also conversion of text to HTML in compose editor was modified in the same way.