Move code for rendering messages/templates → utils

Utils is better structured to handle the logic of what thing to show
for what template type, especially now that what we show for different
template types in different contexts has diverged significantly.

See https://github.com/alphagov/notifications-utils/commit/6b39c1a for
an example of this code moving into utils

Depends on and implements:
https://github.com/alphagov/notifications-utils/pull/84

The main reason for doing this is to get Paul’s fix for the misaligned
CSV columns: https://github.com/alphagov/notifications-utils/pull/87

app/templates/views/send-from-api.html

@@ -1,6 +1,4 @@
 {% extends "withnav_template.html" %}
-{% from "components/email-message.html" import email_message %}
-{% from "components/sms-message.html" import sms_message %}
 {% from "components/api-key.html" import api_key %}
 
 {% block page_title %}
@@ -13,20 +11,7 @@
       API info
   </h1>
 
-  {% if 'email' == template.template_type %}
-    {{ email_message(
-      template.formatted_subject_as_markup,
-      template.formatted_as_markup,
-    ) }}
-  {% elif 'sms' == template.template_type %}
-    <div class="grid-row">
-      <div class="column-two-thirds">
-        {{ sms_message(
-          template.formatted_as_markup,
-        ) }}
-      </div>
-    </div>
-  {% endif %}
+  {{ template.rendered }}
 
   <div class="bottom-gutter">
     {{ api_key(template.id, name="Template ID", thing='template ID') }}