[dagit] Restore schema.graphql to repo
I realized when spinning up vscode today that our eslintrc requires schema.graphql to be present in order to lint GraphQL queries. I don't want anyone to have to run generate-graphql manually after checking the repo out fresh, just to pass lint, so I'm going to put the file back into the repo.
Since Buildkite will still regenerate the GraphQL types, any lint mismatches will still be caught by the build. Engineers might just miss them locally if they fail to regenerate types, which was the case anyway.
Test Plan: Buildkite. Run vscode, open a TS file in dagit/core. Verify that eslint starts up correctly.
Reviewers: bengotow, dgibson
Reviewed By: dgibson
Differential Revision: https://dagster.phacility.com/D8263