WebAlert: fixed HTML-to-text whitespace cleanup

Fixed cleaning of whitespaces when transforming HTML to text.
Consecutive whitespaces are merged into a single space (Eg "<b>1 2</b>" -> "1 2").
Same for whitespaces separated by HTML tag (Eg: "<b>1 </b> 2" -> "1 2").
&nbsp; are preserved (Eg "<b>1&nbsp;&nbsp;&nbsp;2</b>" -> "1 2")