Align loading component in the center. (#535)

* Align loading component in the center.

* Change height of loading component.

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

@@ -1,6 +1,6 @@
 <div class="container-graph">
   <div>
-    <div *ngIf="loading">
+    <div *ngIf="loading" class="loading">
       <div class="text-center">
         <h3 i18n="statistics.loading-graphs">Loading graphs...</h3>
         <br>

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

@@ -41,3 +41,11 @@
     }
   }
 }
+
+.loading{
+  display: flex;
+  flex-direction: column;
+  text-align: center;
+  height: 80vh;
+  justify-content: center;
+}