Refactor to get metadata before validating form

Because we no longer need the form to get the `file_id`, we can get the
metadata before building the form.

This will, in subsequent commits, let us build the form differently
based on the recipient metadata.

Also removed some variables that were assigned to then only used once
and reformatted arguments for readability.

app/main/views/uploads.py

@@ -344,20 +344,25 @@ def send_uploaded_letter(service_id, file_id=None):
     if not (current_service.has_permission('letter') and current_service.has_permission('upload_letters')):
         abort(403)
 
+    metadata = get_letter_metadata(service_id, file_id)
+
+    if metadata.get('status') != 'valid':
+        abort(403)
+
+    recipient_address = metadata.get('recipient')
+
     form = LetterUploadPostageForm()
 
     if not form.validate_on_submit():
         return uploaded_letter_preview(service_id, file_id)
 
-    postage = form.postage.data
-    metadata = get_letter_metadata(service_id, file_id)
-    filename = metadata.get('filename')
-    recipient_address = metadata.get('recipient')
-
-    if metadata.get('status') != 'valid':
-        abort(403)
-
-    notification_api_client.send_precompiled_letter(service_id, filename, file_id, postage, recipient_address)
+    notification_api_client.send_precompiled_letter(
+        service_id,
+        metadata.get('filename'),
+        file_id,
+        form.postage.data,
+        recipient_address,
+    )
 
     return redirect(url_for(
         '.view_notification',