Replace grid-row with govuk-grid-row

Replaced all instances of `grid-row` in the HTML and JavaScript with
`govuk-grid-row`, which is the new GOV.UK Frontend class.

app/templates/views/usage.html

@@ -17,7 +17,7 @@
       {{ pill(years, selected_year, big_number_args={'smallest': True}) }}
     </div>
     <div id='pill-selected-item'>
-      <div class='grid-row'>
+      <div class='govuk-grid-row'>
         <div class='column-one-third'>
           <h2 class='heading-small'>Emails</h2>
           <div class="keyline-block">
@@ -48,7 +48,7 @@
         </div>
       </div>
 
-      <div class='grid-row'>
+      <div class='govuk-grid-row'>
         <div class='column-one-third'>
           <div class="keyline-block">
             &nbsp;
@@ -123,7 +123,7 @@
       {% endif %}
     </div>
 
-    <div class="grid-row">
+    <div class="govuk-grid-row">
       <div class="column-one-third">
         <p class="align-with-heading-copy">
           Financial year ends 31&nbsp;March