Move away from assuming models have a title field

[ababic]

There's an existing and well-used Django convention for controlling the string representation of model instance, and that is to override the __str__() method (something Wagtail's Page model itself does to output the title field value).

There's also a convention for pages in Wagtail for changing the string representation in various parts of the Wagtail admin, which is to override the get_admin_display_title()

Wouldn't it make sense to use either of these by default, rather than assuming models have a title field, and forcing users to specify otherwise when it does not?