Prevent duplicate submissions in sendForm by adding a loading state guard and updating the submit button's disabled condition.

frontend/src/app/pages/forms/appeal/appeal.component.html

@@ -130,7 +130,7 @@
                       }
                     </mat-form-field>
                   </div>
-                  <button mat-raised-button (click)="onSubmit()" [disabled]="form.invalid">
+                  <button mat-raised-button (click)="onSubmit()" [disabled]="formSubmitting || form.invalid">
                     Submit Appeal
                   </button>
                 </section>