'Returns list of content titles for all of self\'s children.
SEQUENCE
VERTICAL
SPLIT_TEST      DISCUSSION
VIDEO A      VIDEO B
Essentially, this function returns a list of display_names (e.g. content titles)
for all of the leaf nodes.  In the diagram above, calling get_content_titles on
SEQUENCE would return the display_names of `VIDEO A`, `VIDEO B`, and `DISCUSSION`.
This is most obviously useful for sequence_modules, which need this list to display
tooltips to users, though in theory this should work for any tree that needs
the display_names of all its leaf nodes.'

def get_content_titles(self):

    if self.has_children:
        return sum((child.get_content_titles() for child in self.get_children()), [])
    else:
        return [self.display_name_with_default_escaped]
