Angular FormItem is using slots instead of ngTemplate for `helpText` and `error`

[ArakTaiRoth]

More Info

Currently our Angular 4.0 implementation relies on the developer to use slots rather than Angular's ngTemplate.

This may not be possible without breaking current implementation, in which case don't do this

Acceptance Criteria:

1. The helpText and error properties should also work with ng-template in Angular, like our other slots
2. helpText and error properties for the Form Item component under Angular v. 4.0+ should be listed as string and TemplateRef
3. The examples using the slotted properties, should be changed to reference ng-template

[ArakTaiRoth]

@ArakTaiRoth chat with @vanessatran-ddi regarding the current implementation