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/navigation.py

@@ -80,6 +80,7 @@ class HeaderNavigation(Navigation):
             'live_services',
             'organisations',
             'platform_admin',
+            'platform_admin_letter_validation_preview',
             'platform_admin_list_complaints',
             'platform_admin_returned_letters',
             'suspend_service',
@@ -442,6 +443,7 @@ class MainNavigation(Navigation):
         'organisation_settings',
         'organisations',
         'platform_admin',
+        'platform_admin_letter_validation_preview',
         'platform_admin_list_complaints',
         'platform_admin_returned_letters',
         'pricing',
@@ -627,6 +629,7 @@ class CaseworkNavigation(Navigation):
         'organisation_settings',
         'organisations',
         'platform_admin',
+        'platform_admin_letter_validation_preview',
         'platform_admin_list_complaints',
         'platform_admin_returned_letters',
         'pricing',
@@ -853,6 +856,7 @@ class OrgNavigation(Navigation):
         'old_using_notify',
         'organisations',
         'platform_admin',
+        'platform_admin_letter_validation_preview',
         'platform_admin_list_complaints',
         'platform_admin_returned_letters',
         'pricing',