refactor: use rxjs pipe syntax (#900)

The `let` operator was not migrated because earlier two reducer needed be refactored
- user-configuration reducer
- present reducer

This commit is prerequisite of the angular upgrade.

packages/uhk-web/src/app/app.component.ts

@@ -4,8 +4,6 @@ import { Observable } from 'rxjs/Observable';
 import { Subscription } from 'rxjs/Subscription';
 import { Action, Store } from '@ngrx/store';
 
-import 'rxjs/add/operator/last';
-
 import { DoNotUpdateAppAction, UpdateAppAction } from './store/actions/app-update.action';
 import { EnableUsbStackTestAction } from './store/actions/device';
 import {