Cascade-delete progress updates

Add a warning about it, and update test to confirm it works

bookwyrm/templates/snippets/components/modal.html

@@ -7,11 +7,10 @@
             </h2>
             {% include 'snippets/toggle/toggle_button.html' with label="close" class="delete" nonbutton=True %}
         </header>
-
         {% block modal-form-open %}{% endblock %}
-
-        {% block modal-body %}{% endblock %}
-
+        <section class="modal-card-body">
+            {% block modal-body %}{% endblock %}
+        </section>
         <footer class="modal-card-foot">
             {% block modal-footer %}{% endblock %}
         </footer>

bookwyrm/templates/snippets/delete_readthrough_modal.html

@@ -1,6 +1,10 @@
 {% extends 'snippets/components/modal.html' %}
 {% block modal-title %}Delete these read dates?{% endblock %}
-
+{% block modal-body %}
+{% if readthrough.progress_updates|length > 0 %}
+You are deleting this readthrough and its {{ readthrough.progress_updates|length }} associated progress updates.
+{% endif %}
+{% endblock %}
 {% block modal-footer %}
 <form name="delete-readthrough-{{ readthrough.id }}" action="/delete-readthrough" method="POST">
     {% csrf_token %}