More robust prop validation with components

[nwinch]

React has some great validation rules for props, and also allows custom validation rules and I think we should utilise this as much possible.

It provides us free, built-in validation for any corrupt data that might get put into the API, or messed up with a migration etc.. and this has been one of the main gripes with Keystone. If we can see data throwing errors in the browser console (or perhaps kick that out to the UI in some way) first thing when entering the admin it could save us hours of time attempting to debug busted data.

Ideally, data will be validated/sanitized before it gets into the API, though if it does pass through the keeper then at least we can verify the dodgy bits easily.

This is especially useful for any props that represent a schema - something like react-prop-schema could be used.