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.
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