Adopt graphql-code-generator

Created by: felixfbecker

This is extended from a subset of https://github.com/sourcegraph/sourcegraph/pull/12284.

It includes:

• Migration to graphql-code-generator from our copy-pasted, buggy generator.
• Change to requestGraphQL() et al to allow them to make use of query types in our application.
• Addition to graphql-code-generator to our Gulp scripts (and dev/start), including watch mode. It uses its native watch mode.
• Unique names for all fragments.
• Docs on how to use the new types (and the preferred way to interact with GraphQL in general), including notes on the "older" types.
• Gitignoring graphql-operations.ts
• Also generating types for the browser extension, not just shared/ and web/

It does not include:

• Refactors of our app away from gql2ts schema-based types. It showed value (I noticed bugs through the higher type safety) but it is a lot more effort.
• Removal of gql2ts, or generating the schema-based types also with graphql-code-genarator. They have slight differences, so this would be more effort, and it makes more sense to jump directly to query-based types when refactoring.