🪲 Fix view programs for non students (#5903)

When trying to access a program from a user that's not in a class, the paged failed due to indexing an empty dictionary.

**How to test**

* Go to  http://localhost:8080/hedy/30bd4e4ce84f40bca40b24b9295e1483/view and it should open normally.

app.py

@@ -1965,7 +1965,7 @@ def view_program(user, id):
                                level=int(result['level']),
                                code=code),
                            is_teacher=user['is_teacher'],
-                           class_id=student_customizations['id'],
+                           class_id=student_customizations.get('id'),
                            next_program_id=next_program_id,
                            next_classmate_program_id=next_classmate_adventure_id,
                            **arguments_dict)

templates/view-program-page.html

@@ -34,9 +34,11 @@ <h1>{{initial_adventure.name}}</h1>
       {% if next_classmate_program_id %}
         <a class="green-btn" href="/hedy/{{ next_classmate_program_id }}/view">{{ _('next_student') }}</a>
       {% endif %}
-      <a class="green-btn" href="/for-teachers/class/{{ class_id }}">
-        {{_('back_to_class')}}
-      </a>
+      {% if class_id %}
+        <a class="green-btn" href="/for-teachers/class/{{ class_id }}">
+          {{_('back_to_class')}}
+        </a>
+      {% endif %}
     {% endif %}
   </div>
   <h2 class="p-0 m-0">{{_('level_title')}} {{level}}</h2>