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/platform-admin/_base_template.html

@@ -5,7 +5,7 @@
 
 {% block maincolumn_content %}
 
-  <div class="grid-row">
+  <div class="govuk-grid-row">
     <div class="column-one-quarter">
       <p class="heading-medium">
         Platform admin

app/templates/views/platform-admin/_global_stats.html

@@ -1,6 +1,6 @@
 {% from "components/big-number.html" import big_number_with_status %}
 {% from "components/message-count-label.html" import message_count_label %}
-<div class="grid-row bottom-gutter">
+<div class="govuk-grid-row bottom-gutter">
   <div class="column-third">
     {{ big_number_with_status(
       global_stats.email.delivered + global_stats.email.failed,

app/templates/views/platform-admin/index.html

@@ -32,7 +32,7 @@
     "open": form.errors | convert_to_boolean
   }) }}
 
-  <div class="grid-row bottom-gutter">
+  <div class="govuk-grid-row bottom-gutter">
     {% for noti_type in global_stats %}
       <div class="column-third">
         {{ big_number_simple(
@@ -53,7 +53,7 @@
     {% endfor %}
   </div>
 
-  <div class="grid-row bottom-gutter">
+  <div class="govuk-grid-row bottom-gutter">
     {% for noti_type in global_stats %}
       <div class="column-third">
         <div class="bordered-text-box">

app/templates/views/platform-admin/returned-letters.html

@@ -9,7 +9,7 @@
 
 {% block platform_admin_content %}
 
-<div class="grid-row">
+<div class="govuk-grid-row">
   <div class="column-two-thirds">
     <h1 class="heading-large">Submit returned letters</h1>
     {% call form_wrapper() %}