Update Register component to remove deprecated code

[craig-dae]

The following console warning shows up on the register component upon updating to Angular 6:

It looks like you're using ngModel on the same form field as formControlName.
Support for using the ngModel input property and ngModelChange event with
reactive form directives has been deprecated in Angular v6 and will be removed
in Angular v7.

For more information on this, see our API docs here:
https://angular.io/api/forms/FormControlName#use-with-ngmodel