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/assets/javascripts/main.js

@@ -1,3 +1,3 @@
 $(() => GOVUK.modules.start());
 
-$(() => new GOVUK.SelectionButtons('.block-label input'));
+$(() => new GOVUK.SelectionButtons('.block-label input, .sms-message-option input'));