Add letter upload form which redirects to blank preview page

Added a form to upload a single letter. Currently this only uses the
form to validate that a file is submitted and that the file is a PDF. If
either of these validations fail, the form will display an error.
Otherwise, we redirect to a new preview page which just has the filename
as the heading for now.

app/templates/views/uploads/preview.html

@@ -0,0 +1,14 @@
+{% extends "withnav_template.html" %}
+{% from "components/page-header.html" import page_header %}
+
+{% block service_page_title %}
+  {{ original_filename }}
+{% endblock %}
+
+{% block maincolumn_content %}
+    {{ page_header(
+        original_filename,
+        back_link=url_for('main.upload_letter', service_id=current_service.id)
+    ) }}
+
+{% endblock %}