Put message previews on ‘Send SMS’ page

A previous commit removed these to differentiate between this page and the
manage templates page. It turns out that we do want previews on this page
because:
- the users for the two pages might be different—they might only be allowed to
  do one or the other depending what permissions they have
- it’s useful to see what the placeholders in the message are before uploading
  a CSV, to make sure the CSV has the correct column headings

This commit re-adds the message preview with a simpler UI. Discussed with
@antimega and we both agreed that the speech bubble tails on the messages should
go.

app/templates/admin_template.html

@@ -55,8 +55,10 @@
             <summary class="dropdown-toggle">
               Service name
             </summary>
-            <a href="#">Switch to A N Other service</a>
-            <a href="{{ url_for('.add_service') }}">Add a new service to GOV.UK Notify</a>
+            <div>
+              <a href="#">Switch to A N Other service</a>
+              <a href="{{ url_for('.add_service') }}">Add a new service to GOV.UK Notify</a>
+            </div>
           </details>
         </div>
         <div class="column-half management-navigation-account">