Remove schema.addToContext

[jasonkuhrt]

Perceived Problem

• schema.addToContext encourages writing middleware where it should NOT be
• schema.addToContext is an API awkwardly between server/schema components
• schema.addToContext functionally overlaps with middleware in some ways

Idea

• Embrace that context is a core data concept of a Nexus app

• Remove it in favour of the proposed middleware system server middleware #523

• Example

   server.middleware((request, response, context) => {
     context.foo = 'bar'
   })

• Global context can be addable like so:

   import { context } from 'nexus'
   
   context.foo = 'bar'