Convert ExternalServiceNode to hooks component

Created by: felixfbecker

Fixes #5240

We can't convert SiteAdminExternalServicesPage at this point because with hooks, the queryExternalServices function that updates the Activation object by definition needs the activation prop as a dep, which means it creates an infinite rerender loop. This situation could be fixed by refactoring the WithActivation HoC to a custom hook, so that the Activation object does not change on every rerender (it's actually perfect for custom hooks).