Add UI for FederationOK

[babolivier]

matrix-org/matrix-federation-tester#76 introduces a top-level FederationOK boolean, which is true as long as no error happened in any check and every attempt to connect to an address succeeded. It might be useful to add a UI for it so the fed-tester-ui can tell people right away wether their federation works fine or there's still something that needs fixing.