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/views/dashboard/_jobs.html

@@ -1,7 +1,7 @@
 {% from "components/table.html" import list_table, field, right_aligned_field_heading, row_heading %}
 {% from "components/big-number.html" import big_number -%}
 
-<div class='dashboard-table'>
+<div class='dashboard-table ajax-block-container'>
   {% call(item, row_number) list_table(
     jobs,
     caption="Recent files uploaded",

app/templates/views/dashboard/_totals.html

@@ -1,7 +1,7 @@
 {% from "components/big-number.html" import big_number_with_status %}
 {% from "components/message-count-label.html" import message_count_label %}
 
-<div class="grid-row">
+<div class="grid-row ajax-block-container">
   <div class="column-half">
     {{ big_number_with_status(
       statistics['email']['requested'],

app/templates/views/dashboard/_usage.html

@@ -1,6 +1,6 @@
 {% from "components/big-number.html" import big_number %}
 
-<div class='grid-row'>
+<div class='grid-row ajax-block-container'>
   <div class='column-half'>
     <div class="keyline-block">
       {{ big_number("Unlimited", 'free email allowance', smaller=True) }}