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

tests/app/main/views/test_platform_admin.py

@@ -758,3 +758,10 @@ def test_platform_admin_submit_empty_returned_letters(mocker, client, platform_a
 
     assert response.status_code == 200
     assert "Can’t be empty" in response.get_data(as_text=True)
+
+
+def test_letter_validation_preview(mocker, client, platform_admin_user):
+    mock_get_user(mocker, user=platform_admin_user)
+    client.login(platform_admin_user)
+    response = client.get(url_for('main.platform_admin_letter_validation_preview'))
+    assert response.status_code == 200