This adds query validation for GraphQL, exposed in
a new raketask (`gitlab:graphql:validate`).

Tests are added for the validation system, which handles common patterns
in our front-end code (such as Apollo client directives, and the use of
ee_else_ce).

The new graphql files used in the tests are excluded from prettier
formatting.

A couple of clearly incorrect graphql files (discovered during testing)
have been fixed. One remaining one has been marked as a known failure.