only change display name if the display name from the current user was changed; add correct display name to new user

Let's remove it, it's also not secure anymore with the introduction of
our API etc...
(And doesn't work with ldap etc…)

Thanks Jenkins!

There was no "isAdminUser()" function which returned bool. This is
irritiating as there were a loooooooot of places in the code which
checked this itself with `OC_Group::inGroup($uid, 'admin)` - why not
use a function for this?
(Especially if you consider that we might change the group name in the
future, which would lead to problems then)

Additionally, @Raydiation needed such a method for his AppFramework :)