Migrate to graphql-code-generator

[kdembler]

Currently, we use apollo-tooling (Apollo Codegen) to generate Typescript typings for all of our queries based on the schema. However, Apollo Codegen seems unmaintained for some time now and there are some issues with it. (generating interfaces instead of types, a lot of types duplication, weird types for some queries/unions). At some point we should switch to https://github.com/dotansimha/graphql-code-generator instead - it's much better maintained and offers much more features. There is a proposal to actually move Apollo Codegen to that library but doesn't seem to be moving forward. More details and possible migration guide here - apollographql/apollo-tooling#2053