Convert Struct Components to Functional

[isosphere]

We are largely using struct components, and most of our components are very simple.

The Yew project recommends functional components as the default:

function components - the recommended way to write components when starting with Yew and when writing simple presentation logic.

I believe that we should update our codebase to consistently use functional components where possible. Perhaps some components require the advanced control struct components can provide, but I doubt we have any such ones today.

I have updated our README.md to indicate a preference for functional components.