Apollo Server 2: Add file uploads

[evans]

This PR adds file uploads to the apollo server 2

• Update CHANGELOG.md with your change (include reference to issue & this PR)
• Make sure all of the significant new logic is covered by tests

[evans]

Needing to add this line is frustrating. When it is removed, the makeExecutableSchema in the constructor fails. We could delay the evaluation of the typeDefs and resolvers until later when this.schema is used with a getter. This feels a bit weird, since the mocks are added in the constructor, so could or could not affect the typeDefs later added to the schema with enhanceSchema

Before the current and past two commits, we had the upload configuration code in the ApolloServer constructor. However fileuploads seem to be a integration specific concern, so I'm pretty convinced that uploads should be configured in registerServer

[evans]

To view the state of the repository when the file upload configuration was done in the constructor, please look at this commit.