Enables GraphQL batch requests

What does this MR do?

Appoloclient supports batching GraphQL queries into a single request using apollo-link-batch-http.

graphql-ruby already supports multiplexing queries. This merge request adds support for that while still enforcing our query limits (depth and complexity).

Batching queries means that all queries can take advantage of loading shared resources using batchloader.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.