Improve areaMutation GQL to support grade context

[vnugent]

Use case

As a contributor I want to set the default grade context for an area.

Todo

• Add grade context to GQL typedef
• Update the corresponding type
• Record grade context object in db.

  openbeta-graphql/src/model/MutableAreaDataSource.ts

  Lines 294 to 297 in cad0ccb

  	if (areaName != null) area.set({ area_name: areaName })
  	if (description != null) area.set({ 'content.description': description })
  	if (shortCode != null) area.set({ shortCode: shortCode.toUpperCase() })
  	if (isDestination != null) area.set({ 'metadata.isDestination': isDestination })