feat: make student card unique per user

core/templates/core/user_preferences.jinja

@@ -35,8 +35,8 @@
     {% endif %}
 
 
-    {% if student_card %}
-      {{ student_card }}
+    {% if student_card_fragment %}
+      {{ student_card_fragment }}
       <p class="justify">
         {% trans %}You can add a card by asking at a counter or add it yourself here. If you want to manually
           add a student card yourself, you'll need a NFC reader. We store the UID of the card which is 14 characters long.{% endtrans %}