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/components/ajax-block.html

@@ -1,10 +1,11 @@
-{% macro ajax_block(partials, url, key, interval=2, finished=False) %}
+{% macro ajax_block(partials, url, key, interval=2, finished=False, form='') %}
   {% if not finished %}
     <div
       data-module="update-content"
       data-resource="{{ url }}"
       data-key="{{ key }}"
       data-interval-seconds="{{ interval }}"
+      data-form="{{ form }}"
       aria-live="polite"
     >
   {% endif %}