Typings cleanup and improvements

(cherry picked from commit b2c43fb2a67965d68d3d35b72302b0cddb5aca23)

frontend/src/App/State/ClientSideCollectionAppState.ts

@@ -0,0 +1,8 @@
+import { CustomFilter } from './AppState';
+
+interface ClientSideCollectionAppState {
+  totalItems: number;
+  customFilters: CustomFilter[];
+}
+
+export default ClientSideCollectionAppState;