feat(FormController): add `$rollbackViewValue` to rollback all controls

Currently it is possible to use `ngModelOptions` to pend model updates until form is submitted, but in case the user wants to reset the form back to its original values he must call `$rollbackViewValue` on each input control in the form. This commit adds a `$rollbackViewValue` on the form controller in order to make this operation easier, similarly to `$commitViewValue`.