Add skeleton in the mining dashboard page

frontend/src/app/components/hashrates-chart-pools/hashrate-chart-pools.component.html

@@ -25,7 +25,7 @@
     </form>
   </div>
 
-  <div *ngIf="hashrateObservable$ | async" [class]="!widget ? 'chart' : 'chart-widget'"
+  <div [class]="!widget ? 'chart' : 'chart-widget'"
     echarts [initOpts]="chartInitOptions" [options]="chartOptions"></div>
   <div class="text-center loadingGraphs" *ngIf="isLoading">
     <div class="spinner-border text-light"></div>