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/api/index.html

@@ -14,7 +14,7 @@
     API integration
   </h1>
 
-  <nav class="grid-row bottom-gutter-1-2">
+  <nav class="govuk-grid-row bottom-gutter-1-2">
     <div class="column-one-third">
       <a class="govuk-link govuk-link--no-visited-state pill-separate-item" href="{{ url_for('.api_keys', service_id=current_service.id) }}">API keys</a>
     </div>
@@ -26,7 +26,7 @@
     </div>
   </nav>
 
-  <div class="grid-row">
+  <div class="govuk-grid-row">
     <div class="column-half">
       <h2 class="heading-small">
         Message log