MOBILE-3253 course: Fix completion's accessibility issue with VoiceOver

Author: nguyenphuctien

src/core/course/components/module-completion/core-course-module-completion.html

@@ -1,3 +1,3 @@
-<a *ngIf="completion" (click)="completionClicked($event)">
+<button *ngIf="completion" (click)="completionClicked($event)">
     <img [src]="completionImage" [alt]="completionDescription">
-</a>
+</button>

src/core/course/components/module-completion/module-completion.scss

@@ -1,10 +1,11 @@
-ion-app.app-root core-course-module-completion a {
+ion-app.app-root core-course-module-completion button {
     display: block;
+    background-color: transparent;
 
     img {
         padding: 5px;
         width: 30px;
         vertical-align: middle;
         max-width: none;
     }
-}
+}