Add letter validation preview functionality

- add get/post view
- create a pdf upload form
- add a template where user can upload the file
- check boundaries of the letter by calling template-preview
- display banner messages with boundaries validation result
- display pages of the document, with visible boundaries overlay
if the document did not pass validation, and without overlay
if they do pass validation

app/templates/components/file-upload.html

@@ -3,7 +3,9 @@
   action=None,
   button_text="Choose file",
   alternate_link=None,
-  alternate_link_text=None
+  alternate_link_text=None,
+  hint=None
+
 ) %}
   <form method="post" enctype="multipart/form-data" {% if action %}action="{{ action }}"{% endif %} class="{% if field.errors %}form-group-error{% endif %}" data-module="file-upload">
     <label class="file-upload-label" for="{{ field.name }}">