'required' Common Field Property

[brownhoward]

User Story

As a Form Designer, I want to specify which elements of a form defined using Form Language are required, so the form renderer can perform client-side data validation.

Acceptance Criteria

1. In the Form Model describing the form, I can set a 'required' property on any form data element to specify is the user is required to enter a value for the element. The default is 'not required'.
2. When the form is rendered based on the Form Model, if the user attempts to leave the data element blank, an error message is displayed.