Add letter upload form which redirects to blank preview page

Added a form to upload a single letter. Currently this only uses the
form to validate that a file is submitted and that the file is a PDF. If
either of these validations fail, the form will display an error.
Otherwise, we redirect to a new preview page which just has the filename
as the heading for now.

app/navigation.py

@@ -304,6 +304,8 @@ class HeaderNavigation(Navigation):
         'template_history',
         'template_usage',
         'trial_mode',
+        'upload_letter',
+        'uploaded_letter_preview',
         'uploads',
         'usage',
         'view_job',
@@ -365,6 +367,8 @@ class MainNavigation(Navigation):
             'view_template_versions',
         },
         'uploads': {
+            'upload_letter',
+            'uploaded_letter_preview',
             'uploads',
         },
         'team-members': {
@@ -859,6 +863,8 @@ class CaseworkNavigation(Navigation):
         'two_factor_email_sent',
         'update_email_branding',
         'update_letter_branding',
+        'upload_letter',
+        'uploaded_letter_preview',
         'uploads',
         'usage',
         'usage_for_all_services',
@@ -1138,6 +1144,8 @@ class OrgNavigation(Navigation):
         'two_factor_email_sent',
         'update_email_branding',
         'update_letter_branding',
+        'upload_letter',
+        'uploaded_letter_preview',
         'uploads',
         'usage',
         'usage_for_all_services',