It persist temporary personal snippets under
user/:id namespaces temporarily while creating
a upload record to track it. If an user gets removed
while it's still a tmp upload, it also gets removed.
If the tmp upload is sent, the upload gets moved to
personal_snippets/:id as before. The upload record
also gets updated to the new model type as well.
This is now entirely handled by `create_note`:
1. Project snippets prevent `create_note`.
2. Uploads already only support routing for personal snippets.
This simplifies some policies and access checks, too!