Make nav and page headings reflect permissions

> If you have the Edit templates permission but not the Send messages permission, the navigation should read
>
> - Text message templates
> - Email templates
> - Letter templates
>
> The page headings should also read
>
> - Text message templates
> - Email templates
> - Letter templates

respectively

https://www.pivotaltracker.com/story/show/115488955

app/templates/views/letters.html

@@ -7,7 +7,11 @@
 {% block maincolumn_content %}
 
   <h1 class="heading-large">
-    Send letters
+    {% if current_user.has_permissions(['send_letters']) %}
+      Send letters
+    {% else %}
+      Letter templates
+    {% endif %}
   </h1>
 
   <p>