web: add storybook for SiteAdminExternalServicePage
Created by: LawnGnome
As SiteAdminExternalServicePage is tightly coupled to fetching its data from GraphQL, I've used fetch-mock to pre-populate it with interesting external service configurations. I am currently unsure how I actually feel about this.
Unfortunately, I need the full page to be able to render and test the webhook configuration blocks, which is my actual goal, as part of #12139.
@eseliger, I'm particularly interested in your thoughts on this. Is this too icky to merge? Should I refactor the page to make it easier to test the bits I need? And, more broadly, since this is my very first time using storybooks, any other feedback would also be greatly appreciated.