Renaming User#display_name to User#name

It is rather confusing having Person#name and User#display_name and
attempting to keep them synchronized.

Factoring to a singular User#name.

This also revealed that the include order of our User models is rather
jumbled up. Namely we should have a User model as:

    class User
      include Hydra::User
      include Sufia::User
      include Curate::UserBehavior
    end

Closes #291
Merge pull request #293 from projecthydra/curate-291

Renaming User#display_name to User#name