Move get_user_display_name() into SODARUser model, deprecate

[mikkonie]

Another case of an unneceessary utility method. This can go in the model. However, this is a breaking change as other sites may be using the method. Also, the implementation should be improved

Tasks

• Move get_user_display_name() into SODARUser.get_display_name()
• Improve implementation to use SODARUser.get_full_name() instead of name
• Deprecate get_user_display_name() with a warning