Make sure partials always have a wrapping <div>

The diffDOM Javascript sometimes throws an error if it can’t calculate
a diff between the original content of the page and the updated HTML
delivered via AJAX. The problem seems to be when there’s not one,
consistent top-level element for it to base its calculations on.

This commit:
- makes sure that all AJAX-delivered partials have a wrapping `<div>`
- that this `<div>` has a consistent class name to make it clear why
  it’s there

app/templates/partials/jobs/count.html

@@ -1,5 +1,5 @@
 {% from "components/pill.html" import pill %}
 
-<div class="bottom-gutter">
+<div class="bottom-gutter ajax-block-container">
   {{ pill('Status', counts, request.args.get('status', '')) }}
 </div>