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 }}">

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 }}">

app/templates/views/platform-admin/letter-validation-preview.html

@@ -0,0 +1,38 @@
+{% extends "views/platform-admin/_base_template.html" %}
+{% from "components/banner.html" import banner %}
+{% from "components/textbox.html" import textbox %}
+{% from "components/file-upload.html" import file_upload %}
+{% from "components/page-footer.html" import page_footer %}
+
+{% block per_page_title %}
+  {{ page_title|capitalize }}
+{% endblock %}
+
+{% block platform_admin_content %}
+
+<div class="grid-row">
+  <div class="column-whole">
+    <h1 class="heading-large">Letter Validation Preview</h1>
+
+    {{ file_upload(
+        form.file,
+        action = url_for('main.platform_admin_letter_validation_preview'),
+        button_text='Upload a PDF document',
+    )}}
+  </div>
+  <div class="column-whole template-container">
+    {% if result %}
+      {{ banner(message, with_tick=True) }}
+    {% elif message %}
+      {{ banner(message, 'dangerous')}}
+    {% endif %}
+
+    {% for page in pages %}
+    <div class="letter">
+      <img src="data:image/png;base64,{{ page }}">
+    </div>
+    {% endfor %}
+  </div>
+</div>
+
+{% endblock %}