Add 'Uploads' hub and navigation

The uploads hub is just a page with text for now - there are no actions
available on the page. It is linked to from a new 'Uploads' menu item on
the left of the page which is only visible if your service has the
`letter` and `upload_letters` permissions and if the current user has
permissions to send messages.

app/templates/views/uploads/index.html

@@ -0,0 +1,16 @@
+{% extends "withnav_template.html" %}
+{% from "components/page-header.html" import page_header %}
+
+{% block service_page_title %}
+  Uploads
+{% endblock %}
+
+{% block maincolumn_content %}
+<div class="grid-row">
+  <div class="column-two-thirds">
+    {{ page_header('Uploads') }}
+
+    <p>Upload a letter and Notify will print, pack and post it for you.</p>
+  </div>
+</div>
+{% endblock %}