Use ‘pence’ in full

This matches what we now do on the product page.

From the GOV.UK style guide:
> Write out pence in full: calls will cost 4 pence per minute from a landline.

– https://www.gov.uk/guidance/style-guide/a-to-z-of-gov-uk-style#money

app/templates/views/pricing.html

@@ -26,7 +26,7 @@
         <li>Up to 306 characters = 2 text messages</li>
         <li>Up to 459 characters = 3 text messages</li>
       </ul></li>
-      <li>Standard text message rate: 1.65p + VAT</li>
+      <li>Standard text message rate: 1.65 pence + VAT</li>
     </ul>
 
     <p>

app/templates/views/usage.html

@@ -37,7 +37,7 @@
           {% if sms_chargeable %}
             {{ big_number(
               sms_chargeable,
-              'at {:.2f}p per message'.format(sms_rate * 100),
+              'at {:.2f} pence per message'.format(sms_rate * 100),
               smaller=True
             ) }}
           {% endif %}