Make a macro for the AJAX update module

This is less repetitive than typing out the HTML with all its attributes
every time.

It also lets us wrap up the idea of ‘finished’ as a parameter, so the
AJAX code will only be initiated when it’s needed, eg if a job is still
processing.

app/templates/components/ajax-block.html

@@ -0,0 +1,15 @@
+{% macro ajax_block(url, key, interval=2, finished=False) %}
+  {% if not finished %}
+    <div
+      data-module="update-content"
+      data-resource="{{ url }}"
+      data-key="{{ key }}"
+      data-interval-seconds="{{ interval }}"
+      aria-live="polite"
+    >
+  {% endif %}
+    {{ caller() }}
+  {% if not finished %}
+    </div>
+  {% endif %}
+{% endmacro %}