Make text message price dependent on date

So we don’t have to deploy a change on a Saturday.

In the future this could pull from the rates in the database, but while
that code is shifting around I didn’t want to touoch it. We’d also have
to think about caching so as not to have a non-authenticated route
hitting the database.

app/templates/views/signedout.html

@@ -153,7 +153,7 @@
           <h3 class="govuk-visually-hidden">Text messages</h3>
           <div class="product-page-big-number">Up to 40,000</div>
           free text messages a year,<br>
-          then 1.72 pence per message
+          then {{ sms_rate }} pence per message
         </div>
         <div class="govuk-grid-column-one-half">
           <h3 class="govuk-visually-hidden">Letters</h3>