Move more of the file-handling logic from UploadService and
StorageManager into MediaAsset. This is part of refactoring posts and
uploads to allow multiple images per post.
Fix bug where pruning uploads failed because the rclone binary couldn't
be found. The upload pruner runs under cron, which has a fixed default
$PATH of "/bin:/usr/bin", but in production rclone is installed under
/usr/local/bin. This caused the upload pruner to fail, which prevented
the rest of daily maintenance from running.
