CRM-21335 - crmUi - Allow forcing the required `*` flag

CRM-21335 - crmUi - Allow forcing the required `*` flag

The `crm-ui-field` autodetects whether the element is required by inspecting
the Angular form model. However, in CRM-21335, we have a scenario where this
inspection fails. (There are other possible scenarios -- eg generally, if the
apparent field element is not actually a well-behaved+singular Angular form element.)

This revision adds a new option to make the field *render* as required even
if there's no underlying form ngModel element.

Ex:

```html
<div crm-ui-field="{name: 'subform.recipients', title: ts('Recipients'), required: true}">...</div>
```