NOJIRA: Fix target blank contents

In plain text mesages contents, URL are "rewrote" to be displayed with a HTML anchor and target="_new" attribute. But in messages with HTML contents, HTML anchors are not "rewrote" to add the target="_new" attribute. So, all HTML anchors have different opening policy depending on target attribute.

To fix the problem I created a static method in MessageUtils to add the target attribute if not present on all HTML anchors. I also provide a unit test to check the job.