Fix 'Discard changes?' when updating external service

Created by: mrnugget

This fixes the 'Discard changes?' confirmation dialog that pops up when the user saves the external service config.

It also refactors the code to use useQuery and useMutation hooks (which are recommended by frontend-platform team), which makes the logic a bit simpler and more consistent with rest of codebase.

Test plan

• Storybook
• Manual testing (with modified graphql backend to check for warnings)

App preview:

• Web
• Storybook

Check out the client app preview documentation to learn more.