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.
