Add plural to image/document confirm delete string

This stops it being flagged as a duplicate with the bulk delete template, and means both templates can share the string.

wagtail/documents/templates/wagtaildocs/documents/confirm_delete.html

@@ -14,7 +14,14 @@
             </div>
         {% endif %}
 
-        <p>{% trans "Are you sure you want to delete this document?" %}</p>
+        <p>
+            {# This message will only appear in the singular, but we specify a plural so it can share the translation string with confirm_bulk_delete.html #}
+            {% blocktrans trimmed count counter=1 %}
+            Are you sure you want to delete this document?
+            {% plural %}
+            Are you sure you want to delete these documents?
+            {% endblocktrans %}
+        </p>
         <form action="{% url 'wagtaildocs:delete' document.id %}" method="POST">
             {% csrf_token %}
             <input type="hidden" value="{{ next }}" name="next">

wagtail/images/templates/wagtailimages/images/confirm_delete.html

@@ -17,7 +17,14 @@
                     <a href="{{ image.usage_url }}">{% blocktrans count usage_count=image.get_usage.count %}Used {{ usage_count }} time{% plural %}Used {{ usage_count }} times{% endblocktrans %}</a>
                 </div>
             {% endif %}
-            <p>{% trans "Are you sure you want to delete this image?" %}</p>
+            <p>
+                {# This message will only appear in the singular, but we specify a plural so it can share the translation string with confirm_bulk_delete.html #}
+                {% blocktrans count counter=1 %}
+                    Are you sure you want to delete this image?
+                {% plural %}
+                    Are you sure you want to delete these images?
+                {% endblocktrans %}
+            </p>
             <form action="{% url 'wagtailimages:delete' image.id %}" method="POST">
                 {% csrf_token %}
                 <input type="hidden" value="{{ next }}" name="next">