MDL-35104 auth: Don't allow modification of username

MDL-35104 auth: Don't allow modification of username

Presently it is either unreliable, or not possible to change
the username of a user created with an authentication plugin.
In some cases it is even hard coded to fail. Ideally we would
sync the username, and the issue MDL-21928 exists to address
that. However, in the mean time we should not allow the
username of an external user to be modified.