[WIP][DISCUSSION] Form required fields

@moneyadviceservice/frontend 

I've added in a required field indicator to the form examples. With my implementation, NVDA and JAWS announce the required field twice.

Our options:

1. CURRENT *visually-hidden* and *aria-required*. Supported by legacy/current/future ATs by utilising visually hidden text and the official aria attribute. "required" announced twice.

2. *visually-hidden*. Supported by legacy/current/future ATs by using visually hidden text. "required" announced once.

3. *aria-required*. Supported only by current/future ATs with ARIA support. "required" announced once.