only show external link icon for external links

pdb-js/src/app/dashboard-page/dashboard/add-text-dialog/add-text-dialog.component.html

@@ -9,7 +9,7 @@
     <mat-label>Text</mat-label>
     <textarea matInput [(ngModel)]="text" #textElement focus></textarea>
   </mat-form-field>
-  <div>Text field supports <a href="https://spec.commonmark.org/" target="_blank" rel="noopener">Markdown</a>.</div>
+  <div>Text field supports <a href="https://spec.commonmark.org/" class="external-link" target="_blank" rel="noopener">Markdown</a>.</div>
 </div>
 <div mat-dialog-actions align="end">
   <button mat-button mat-dialog-close (click)="close()">Cancel</button>