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/support/index.html

@@ -9,7 +9,7 @@
 
 {% block maincolumn_content %}
 
-<div class="grid-row">
+<div class="govuk-grid-row">
   <div class="column-two-thirds">
 
     <h1 class="heading-large">Support</h1>
@@ -21,11 +21,11 @@
     {% endcall %}
 
     <p>You can also <a class="govuk-link govuk-link--no-visited-state" href="https://ukgovernmentdigital.slack.com/messages/C0E1ADVPC">contact us on Slack</a>.</p>
-    
+
     <h2 class="heading-medium">Office hours</h2>
     <p>Our office hours are 9:30am to 5:30pm, Monday to Friday.</p>
     <p>When you report a problem in office hours, we’ll aim to read it within 30 minutes and reply within one working day.</p>
-    
+
     <h2 class="heading-medium">Out-of-hours</h2>
     <p>Outside office hours, response times depend on whether you’re reporting an emergency.</p>
     <p>If it’s an emergency, we’ll reply within 30 minutes and update you every hour until the problem’s fixed.</p>