This change is needed as
a) there are many different types of notifications. A notification about a new comment does not only relate the comment but also adoption notice and a user that made the comment. Therefore, I introduce a new general notification that has optional links to other objects.
b) this change allows to define a different representation of e-mails vs. display on the website. The new notification type is in charge of representation from now on. Title and text will be deprecated and eventually removed
