User admin forms have inconsistent behaviour with KVP that duplicate existing user properties

[JimKerslake]

Spinning this out from #12

At the moment the UI can go a bit awry if you try to declare KVP properties that duplicate existing native User fields.

This comes down to a difference between how the administrator's User Management forms handle things -
'New User' behaves differently from 'Manage' (= edit user).

For the 'Manage' (user edit) screen, you can get the same form input element twice (eg with DoB).

However, if you get rid of that by setting the 'VisibleOnAdminUserEdit' and 'EditableOnAdminUserEdit' config settings to false -
but if you have declared the KVP for DoB as 'required' -
then the admin's "New User" screen becomes impossible to submit, because it does not have its own pre-existing DoB input, the above config prevents the KVP DoB form input element being rendered, but the validation still kicks in.

So maybe a bit of work needed on those various user management forms, validation etc.
(But keeping in mind also the user self-registration form, which is also affected by KVP).