Add support for new GitHub check suite states

Created by: eseliger

GitHub added new states, so we should add them to the parsing function here: https://github.com/sourcegraph/sourcegraph/blob/main/enterprise/internal/batches/state/state.go#L335

// Schema Changes for 2020-02-05
// Enum value STALE was added to enum CheckConclusionState
// Enum value SKIPPED was added to enum CheckConclusionState