Make AJAX requests on activity page POST not GET

See parent commit for the reason we’re doing this.

Currently our AJAX requests only work as `GET` requests. So this commit
does a bit of work to make them work as `POST` requests. This is
optional behaviour, and will only happen when the element in the page
that should be updated with AJAX has the `data-form` attribute set. It
will take the form that has the corresponding `id`, serialise it, and
use that data as the body of the post request. If not form is specified
it will not do the serialisation, and submit as a `GET` request as
before.

app/templates/views/notifications.html

@@ -24,9 +24,9 @@
     method="post"
     action="{{ url_for('.view_notifications', service_id=current_service.id, message_type=message_type) }}"
     class="grid-row"
+    id="search-form"
   >
     <div class="column-three-quarters">
-      <input type="hidden" name="status" value="{{ status }}">
       {{ textbox(
         search_form.to,
         width='1-1',
@@ -41,8 +41,9 @@
 
   {{ ajax_block(
     partials,
-    url_for('.get_notifications_as_json', service_id=current_service.id, message_type=message_type, status=status, page=page, to=to),
-    'notifications'
+    url_for('.get_notifications_as_json', service_id=current_service.id, message_type=message_type, status=status, page=page),
+    'notifications',
+    form='search-form'
   ) }}
 
 {% endblock %}