Put upload letters button on jobs page

This is going to become the one true ‘Uploads’ page, so it need the
sticky footer that takes users into the new upload letters journey.

app/models/service.py

@@ -462,6 +462,10 @@ class Service(JSONModel):
             key=lambda folder: folder['name'].lower(),
         )
 
+    @property
+    def can_upload_letters(self):
+        return self.has_permission('letter') and self.has_permission('upload_letters')
+
     @cached_property
     def all_template_folder_ids(self):
         return {folder['id'] for folder in self.all_template_folders}