refactor(@core): refactor data services for better integration (#1997)

With this change all components, which used data services before, now use abstract classes of service interfaces, mock services extend interface services, CoreModule contains code to inject a needed implementation of some service.

src/app/@core/data/user-activity.ts

@@ -0,0 +1,12 @@
+import { Observable } from 'rxjs';
+
+export interface UserActive {
+  date: string;
+  pagesVisitCount: number;
+  deltaUp: boolean;
+  newVisits: number;
+}
+
+export abstract class UserActivityData {
+  abstract getUserActivityData(period: string): Observable<UserActive[]>;
+}