Moving chart loading spinner to chart component

fixes #885

frontend/src/app/components/television/television.component.html

@@ -1,10 +1,5 @@
 <div id="tv-wrapper">
-
-  <div *ngIf="mempoolStats.length === 0" class="loading">
-    <div class="spinner-border text-light"></div>
-  </div>
-
-  <div class="tv-container" *ngIf="mempoolStats.length">
+  <div class="tv-container">
     <div class="chart-holder">
       <app-mempool-graph
         [template]="'advanced'"
@@ -12,7 +7,7 @@
         [height]="600"
         [left]="60"
         [right]="10"
-        [data]="mempoolStats"
+        [data]="mempoolStats && mempoolStats.length ? mempoolStats : null"
         [showZoom]="false"
       ></app-mempool-graph>
     </div>

frontend/src/app/components/television/television.component.scss

@@ -16,6 +16,7 @@
 }
 
 .chart-holder {
+  position: relative;
   height: 650px;
   width: 100%;
   margin: 30px auto 0;