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/choose-file.html

@@ -0,0 +1,28 @@
+{% extends "withnav_template.html" %}
+{% from "components/file-upload.html" import file_upload %}
+{% from "components/page-header.html" import page_header %}
+
+{% block service_page_title %}
+  Upload a letter
+{% endblock %}
+
+{% block maincolumn_content %}
+<div class="grid-row">
+  <div class="column-five-sixths">
+    {{ page_header(
+        'Upload a letter',
+        back_link=url_for('main.uploads', service_id=current_service.id)
+    ) }}
+
+  <p>
+    {{ file_upload(
+      form.file,
+      action = url_for('main.upload_letter', service_id=current_service.id),
+    )}}
+  </p>
+  <p>You can upload a single letter as a PDF.</p>
+  <p>Your file must meet our <a href="https://docs.notifications.service.gov.uk/documentation/images/notify-pdf-letter-spec-v2.3.pdf">letter specification</a>.</p>
+
+  </div>
+</div>
+{% endblock %}

app/templates/views/uploads/index.html

@@ -7,10 +7,12 @@
 
 {% block maincolumn_content %}
 <div class="grid-row">
-  <div class="column-two-thirds">
+  <div class="column-five-sixths">
     {{ page_header('Uploads') }}
 
     <p>Upload a letter and Notify will print, pack and post it for you.</p>
+
+    <a href="{{ url_for('.upload_letter', service_id=current_service.id) }}" class="button-secondary">Upload a letter</a>
   </div>
 </div>
 {% endblock %}

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 %}