Refactor withInjector away from the legacy API

[ScopeyNZ]

Currently withInjector is using the context API that was included with React 15. In React 16 this API was deprecated and replaced in React 16.3 with the new context API:

New API: https://reactjs.org/docs/context.html
Legacy API: https://reactjs.org/docs/legacy-context.html

I see two options:

• Refactor withInjector to be a more standard HOC that supplies some resolver function as a prop
• Refactor withInjector to use the new Context API in React 16.3 (which means this issue will be reliant on the upgrade covered in Upgrade to React 16 and latest node.js version #648)

I ran a quick usage scan on our own modules - I can only see it used by admin (outside of other Injector library code) in FormBuilder.