This restores the previous functionality. The script now allows the following options:
* `discourse backup` (uses the system generated filename)
* `discourse backup <some_filename>` (uses the provided filename)
* `discourse backup </some/path/to/filename>` (moves the backup to the provided path with the given filename)
Remote backup stores do not support the last option.
Some file extensions (like `.tar.gz`) are automatically removed from the provided filename.
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