Add recipient to message previews

When previewing a template on the send page, having the recipient appear
as a placeholder should help reinforce the relationship between the
columns in the CSV and the placeholders.

Then, when previewing a message, having the template populated with the
first recipient’s email address/phone number should reinforce that
relationship again.

app/templates/components/email-message.html

@@ -1,4 +1,13 @@
-{% macro email_message(subject, body, name=None, edit_link=None, from_name=None, from_address=None) %}
+{% macro email_message(
+  subject,
+  body,
+  name=None,
+  edit_link=None,
+  from_name=None,
+  from_address=None,
+  recipient=None,
+  show_placeholder_for_recipient=False
+) %}
   {% if name %}
     <h3 class="email-message-name">
       {% if edit_link %}
@@ -20,6 +29,20 @@
               </td>
             </tr>
           {% endif %}
+          {% if recipient is not none %}
+            <tr>
+              <th>To</th>
+              <td>
+                {% if show_placeholder_for_recipient %}
+                  <span class="placeholder">
+                    email address
+                  </span>
+                {% else %}
+                  {{ recipient }}
+                {% endif %}
+              </td>
+            </tr>
+          {% endif %}
           {% if subject %}
             <tr class="email-message-meta">
               <th>Subject</th>