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.
Rephrase editor warning to better match both ways text2html and html2text
Improve _convert_line() performance
Fix flowed lines recognition
Disable wrapping non-flowed lines on dash character
Fix invalid closing tag
Merge branch 'dev-text2html' of github.com:roundcube/roundcubemail into dev-text2html
Bring back the old behaviour where text messages without format=flowed are auto-wrapped.
Make it the default in text2html class.
Improved handling of new-lines in quoted paragraphs on text2html conversion
Merge branch 'dev-text2html'

Conflicts:
	program/js/app.js

Fix handling magic_quotes in rcube_text2html and rcube_html2text - move
stripslashes from these classes to action files
Update changelog