feat: improve button style of modal tos

The "Accept new terms of service" button on the Modal
show when the terms of service is updated, improve the
style of the button.

relates to fccn/nau-technical#81

edx-platform/nau-basic/lms/templates/modal-update-terms-of-service.html

@@ -20,7 +20,7 @@
 <div class="modal-update-terms-of-service-container" style="display: none;"> 
   <div class="modal-update-terms-of-service-content">
     ${modal_tos_text | n}
-    <button class="button-modal btn-primary">${_('Accept new terms of service')}</button>
+    <button class="btn btn-primary button-modal">${_('Accept new terms of service')}</button>
   </div>
 </div>