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

@@ -11,7 +11,8 @@
   <h1 class="heading-large">
     Letter contact details
   </h1>
-  <form method="post">
+  <div class="grid-row">
+    <form method="post" class="column-half">
       {{ textbox(
         form.letter_contact_block,
         width='1-1',
@@ -23,5 +24,6 @@
         back_link_text='Back to settings'
       ) }}
     </form>
+  </div>
 
 {% endblock %}