feat: add progress bar (#66)

Add progress bar for loading data via http requests.

Fixes #65

src/app/core/progress-bar.service.ts

@@ -0,0 +1,35 @@
+import { Injectable, EventEmitter } from '@angular/core';
+
+@Injectable({
+  providedIn: 'root'
+})
+export class ProgressBarService {
+
+  public updateProgressBar: EventEmitter<any>;
+  private requestsRunning = 0;
+
+  constructor() {
+    this.updateProgressBar = new EventEmitter();
+  }
+
+  public getRequestsRunning(): number {
+    return this.requestsRunning;
+  }
+
+  public increase(): void {
+    this.requestsRunning++;
+    if (this.requestsRunning === 1) {
+      this.updateProgressBar.emit('indeterminate');
+    }
+  }
+
+  public decrease(): void {
+    if (this.requestsRunning > 0) {
+      this.requestsRunning--;
+      if (this.requestsRunning === 0) {
+        this.updateProgressBar.emit('none');
+      }
+    }
+  }
+
+}