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.
This commit extends the `ajax_block` component to take a `dict` of
partials, from which it can select the partial matching its `key`
argument and print its HTML to the page.
This means that the same markup is only rendered in one place, rather
than in two (individually in the JSON endpoint and as `include`s in the
parent template).
This is less repetitive than typing out the HTML with all its attributes
every time.
It also lets us wrap up the idea of ‘finished’ as a parameter, so the
AJAX code will only be initiated when it’s needed, eg if a job is still
processing.
