Display broadcasts without a template

At the moment the admin app expects all broadcasts to have a template,
and expects the content of the alert to come from the template.

This commit makes it so those pages can still get a `Template` instance,
but populated with content straight from the `content` field in the
database.

app/templates/views/broadcast/preview-areas.html

@@ -21,7 +21,7 @@
 
   {{ page_header(
     "Choose where to broadcast to",
-    back_link=url_for('.view_template', service_id=current_service.id, template_id=broadcast_message.template_id)
+    back_link=back_link
   ) }}
 
   {% for area in broadcast_message.areas %}