Name Handling for users

Currently, we store firstname and lastname and do some magic with infixes, if present. However, internationally, names do not nicely fit in such a format.

We should switch from using a firstname, lastname and title to using the displayname. I think this should be sufficient for all we need to do. This will require changes in lots of places, from the external user registration form to the searching options in the management panel. Ideally we also change all locations referring to "full_name" to "display_name".