Add a cancel job button

If you schedule a job you might change your mind or circumstances might
change. So you need to be able to cancel it. This commit adds a button
on the job page which hits the `…/cancel` API endpoint for a job.

app/templates/partials/jobs/notifications.html

@@ -1,4 +1,5 @@
 {% from "components/table.html" import list_table, field, right_aligned_field_heading, date_field, row_heading %}
+{% from "components/page-footer.html" import page_footer %}
 
 <div class="ajax-block-container">
   {% if job.job_status == 'scheduled' %}
@@ -6,6 +7,14 @@
     <p>
       Sending will start at {{ job.scheduled_for|format_time }}
     </p>
+    <div class="page-footer">
+      <form method="post">
+        {{ page_footer(
+          button_text="Cancel sending",
+          destructive=True
+        ) }}
+      </form>
+    </div>
 
   {% else %}