MDL-27953 introduced a new auth plugin method: can_be_manually_set() which prevents users from manually uploading users and setting the auth method to that.

I think this should be respected in manual individual user creation too, not just bulk user upload.

Steps to reproduce:

1. As admin
2. Go to admin > users > add new user
3. Select an auth method for the new user

Expected results:
The auth plugin with can_be_manually_set() to false should not be present (e.g. shibboleth

Actual results:
A user can be created with this auth method manually, but not in bulk user upload.