Improve readability of notifications by sending HTML mails

Notifications are formatted using plain text mails since day one. The downside of using this simple format is that URL's are visible as full URL's and greatly reduce the readability of the mail. A mail containing several news updates is almost unreadable.

For some time now we are capable of sending HTML mail. For notification this has not been adjusted. This goal is to use HTML code instead of plain text mails and to improve formatting of the notification mails.