And move UntrackedFile into PopulateUntrackedUploadsDependencies, and move its spec into its own file.

Specifically, `PopulateUntrackedUploads` and its spec.

And use :migration tag to use deletion strategy, and to avoid caching tables, and to lock into a particular schema.

Attempting to fix intermittent spec errors `PG::UndefinedTable: ERROR:  relation "public.untracked_files_for_uploads" does not exist`.

To fix for MySQL.

It makes a debugging slightly easier, but is not necessary, and is a waste of resources.

Since `calculate_checksum` depends on `Uploader` classes which are not defined in this background migration and may change at any time.

So the path on source installs cannot be too long for our column.

And fix the column length test since Route.path is limited to 255 chars, it doesn’t matter how many nested groups there are.

by copy-pasting in the whole `Upload` class.

Also, fix `Namespace` `model_type` (it should not be `Group`).

To process the unhashed_upload_files table.