Image width prop should be optional

[pjlee11]

Is your feature request related to a problem? Please describe.
In the psammead-image we currently have props for both height and width. However, height is an optional value due to this issue and width is required making them inconsistent. For context <amp-img> requires both height and width to be AMP valid. This means in some cases height and width are required and in other cases they aren't strictly required HTML attributes on the element.

Also we are aiming for our psammead components to be as re-usable as possible meaning we shouldn't enforce width/height being passed in to the component but instead enforce this in the container bespoke to our rendering scenario.

Describe the solution you'd like

• Make width prop optional on the image component
• Explain the benefits of passing width and height props in the psammead-image README.md

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Testing notes
[Tester to complete]

Dev insight: There should be no regression in the component. There should be stories and unit test for the optional behaviour of the width prop.

Additional context

• Initially labelled with "Refinement needed"