Send email notifications as HTML

Since task details & comments often contain HTML, it is much more useful if the email notifications render fully.  This is done with 3 changes:
1) Set the content type to text/html
2) Preserve formatting of non-HTML portions (e.g. heading, disclaimer, etc.) by converting newlines into HTML line breaks
3) Upconvert plaintext URLs into hyperlinks (useful mainly for links back to Flyspray)