[graphiql] Option to have fixed theme and disable settings dialog

When using the `` component there is an option for `editorTheme` which doesn't work how I would've expected. I did some searching and found that you can use the `useTheme` hook to specify a theme:

```tsx
useEffect(() => {
setTheme("light");
// eslint-disable-next-line react-hooks/exhaustive-deps
}, []);
```

For my purposes I have also disabled headers with `isHeadersEditorEnabled={false}` which renders the settings button redundant. It would be great if there was a way to hide it to avoid confusion.