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/error/401.html

@@ -1,7 +1,7 @@
 {% extends "withoutnav_template.html" %}
 {% block per_page_title %}You’re not authorised to see this page{% endblock %}
 {% block maincolumn_content %}
-  <div class="grid-row">
+  <div class="govuk-grid-row">
     <div class="column-two-thirds">
     <h1>You’re not authorised to see this page</h1>
       <p><a class="govuk-link govuk-link--no-visited-state" href="{{ url_for('.sign_in' )}}">Sign in</a> to GOV.UK Notify and try again.</p>

app/templates/error/403.html

@@ -1,7 +1,7 @@
 {% extends "withoutnav_template.html" %}
 {% block per_page_title %}You’re not allowed to see this page{% endblock %}
 {% block maincolumn_content %}
-    <div class="grid-row">
+    <div class="govuk-grid-row">
       <div class="column-two-thirds">
         <h1 class="heading-large">
           You’re not allowed to see this page

app/templates/error/404.html

@@ -1,7 +1,7 @@
 {% extends "withoutnav_template.html" %}
 {% block per_page_title %}Page not found{% endblock %}
 {% block maincolumn_content %}
-    <div class="grid-row">
+    <div class="govuk-grid-row">
       <div class="column-two-thirds">
         <h1 class="heading-large">
           Page not found

app/templates/error/410.html

@@ -1,7 +1,7 @@
 {% extends "withoutnav_template.html" %}
 {% block per_page_title %}Page not found{% endblock %}
 {% block maincolumn_content %}
-    <div class="grid-row">
+    <div class="govuk-grid-row">
       <div class="column-two-thirds">
         <h1 class="heading-large">
           Page not found

app/templates/error/413.html

@@ -1,12 +1,12 @@
 {% extends "withoutnav_template.html" %}
 {% block per_page_title %}File too big{% endblock %}
 {% block maincolumn_content %}
-    <div class="grid-row">
+    <div class="govuk-grid-row">
       <div class="column-two-thirds">
         <h1 class="heading-large">
           The file you uploaded was too big
         </h1>
-        <div class="grid-row">
+        <div class="govuk-grid-row">
           <div class="column-two-thirds">
             <p>
               Files must be smaller than 5 MB.

app/templates/error/500.html

@@ -1,7 +1,7 @@
 {% extends "withoutnav_template.html" %}
 {% block per_page_title %}Sorry, there’s a problem with the service{% endblock %}
 {% block maincolumn_content %}
-  <div class="grid-row">
+  <div class="govuk-grid-row">
     <div class="column-two-thirds">
       <h1 class="heading-large">
         Sorry, there’s a problem with GOV.UK Notify