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/styleguide.html

@@ -24,7 +24,7 @@
   </p>
 
   <h2 class="heading-large">Banner</h2>
-  <div class="grid-row">
+  <div class="govuk-grid-row">
     <div class="column-three-quarters">
       <p>Used to show the status of a thing or action.</p>
 
@@ -42,7 +42,7 @@
 
   <p>Used to show some important statistics.</p>
 
-  <div class="grid-row">
+  <div class="govuk-grid-row">
     <div class="column-one-third">
       {{ big_number("567") }}
     </div>
@@ -109,7 +109,7 @@
   ) }}
 
   <h2 class="heading-large">Tables</h2>
-  <div class="grid-row">
+  <div class="govuk-grid-row">
     <div class="column-three-quarters">
       <p>
         Used for comparing rows of data.