Convert store selectors to Typescript

(cherry picked from commit a57b35a1966266b49d1241474fe3b69523f70474)

frontend/src/App/State/SystemAppState.ts

@@ -0,0 +1,10 @@
+import SystemStatus from 'typings/SystemStatus';
+import { AppSectionItemState } from './AppSectionState';
+
+export type SystemStatusAppState = AppSectionItemState<SystemStatus>;
+
+interface SystemAppState {
+  status: SystemStatusAppState;
+}
+
+export default SystemAppState;