Send adminUsername in /settings again

iOS app builds before Companion 2c9cc1c cannot decode /settings without
it, and App Review still tests such builds. A test keeps it in place.

api.go

@@ -1001,6 +1001,9 @@ func (a *App) getSettings(w http.ResponseWriter, r *http.Request) {
 		"geo":         a.geoStatus(),
 		"fingerprint": a.tls.Fingerprint(),
 		"logPath":     a.logPath,
+		// Required by iOS app builds before 2c9cc1c, which App Review
+		// still tests.
+		"adminUsername": a.username(cfg, who(r).UserID),
 	})
 }