fix: wrong text on member form submit button

club/templates/club/fragments/add_member.jinja

@@ -36,7 +36,7 @@
     </fieldset>
     <button type="submit" class="btn btn-blue">
       <i class="fa fa-user-plus"></i>
-      {%- if club.user -%}
+      {%- if form.user -%}
         {% trans %}Add{% endtrans %}
       {%- else -%}
         {% trans %}Join{% endtrans %}