Allow deleting default letter contact blocks

It’s possible to delete default letter contact blocks because there is a
fallback – having a blank letter contact block. This is different to SMS
senders and reply to addresses.

For this to make sense it also means:
- adding the ‘blank’ letter contact block to the list of letter contact
  blocks
- having a way of setting the default back to being blank

app/templates/views/service-settings/letter-contact-details.html

@@ -17,11 +17,18 @@
     ) }}
   </div>
   <div class="user-list">
-    {% if not letter_contact_details %}
-      <div class="user-list-item">
-        <span class="hint">You haven’t added any letter contact details yet</span>
-      </div>
-    {% endif %}
+    <div class="user-list-item">
+      <span class="hint">
+        Blank
+        {% if current_service.default_letter_contact_block %}
+          {% if current_user.has_permissions('manage_service') %}
+            <a class="user-list-edit-link" href="{{ url_for('.service_make_blank_default_letter_contact', service_id =current_service.id) }}">Make default</a>
+          {% endif %}
+        {% else %}
+          (default)
+        {% endif %}
+      </span>
+    </div>
     {% for item in letter_contact_details %}
       <div class="user-list-item">
         <p>

app/templates/views/service-settings/letter-contact/edit.html

@@ -24,20 +24,24 @@
       rows=10,
       highlight_tags=True
     ) }}
+
+
     {% if form.is_default.data %}
       <p class="form-group">
-        This is currently your default address for {{ current_service.name }}
+        This is the default address for {{ current_service.name }}
       </p>
     {% else %}
       <div class="form-group">
         {{ checkbox(form.is_default) }}
       </div>
     {% endif %}
+
     {{ page_footer(
       'Save',
       delete_link=url_for('.service_confirm_delete_letter_contact', service_id=current_service.id, letter_contact_id=letter_contact_id),
       delete_link_text='Delete'
     ) }}
+
   {% endcall %}
 
 {% endblock %}