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/views/platform-admin/_base_template.html

@@ -23,6 +23,7 @@
           ('Find users by email', url_for('main.find_users_by_email')),
           ('Email Complaints', url_for('main.platform_admin_list_complaints')),
           ('Returned letters', url_for('main.platform_admin_returned_letters')),
+          ('Letter validation preview', url_for('main.platform_admin_letter_validation_preview')),
         ] %}
           <li>
             <a href="{{ url }}">