fix(admin): loader style

src/app/admin/components/lockdown-info/manage-lockdown-info.component.html

 <div *ngIf="isLoading" class="loader">
-  <img class="loader-gif" src="/assets/gif/loader_circle.gif" alt />
+  <img class="loader-gif" src="/assets/gif/loader_circle_grey.gif" alt />
 </div>
 <div *ngIf="!isLoading" fxLayout="column" fxLayoutAlign="center center">
   <H2>Gestion de l'affichage des infos Covid</H2>