Make letter contact textbox half-wide

This approximates the width of the contact block area in the letter
template itself.

app/templates/views/service-settings/set-letter-contact-block.html

@@ -11,17 +11,19 @@
   <h1 class="heading-large">
     Letter contact details
   </h1>
-  <form method="post">
-    {{ textbox(
-      form.letter_contact_block,
-      width='1-1',
-      rows=10
-    ) }}
-    {{ page_footer(
-      'Save',
-      back_link=url_for('.service_settings', service_id=current_service.id),
-      back_link_text='Back to settings'
-    ) }}
-  </form>
+  <div class="grid-row">
+    <form method="post" class="column-half">
+      {{ textbox(
+        form.letter_contact_block,
+        width='1-1',
+        rows=10
+      ) }}
+      {{ page_footer(
+        'Save',
+        back_link=url_for('.service_settings', service_id=current_service.id),
+        back_link_text='Back to settings'
+      ) }}
+    </form>
+  </div>
 
 {% endblock %}