Investigate BigInt to float conversion for large numbers.

`BigInt` types from the DB are currently converted to `float` in the GQL schema. We should investigate whether the 64-bit limit for python float poses a problem here, and whether or not it's better to switch to a different type.

Source: #371
> maybe a bit off topic here, but wondering about the choice of BigInteger->Float. assume this would break down after 64 bits. any plans for graphql-core to provide a [BigInt type like this](https://www.npmjs.com/package/graphql-bigint)?