D5067: help: assigning categories to existing commands

[Yuya Nishihara]

> +# Add our category before "Repository maintenance".
> +help.CATEGORY_ORDER.insert(
> +    help.CATEGORY_ORDER.index(command.CATEGORY_MAINTENANCE),
> +    _HELP_CATEGORY)
> +help.CATEGORY_NAMES[_HELP_CATEGORY] = 'GPG signing'

Moved this to extsetup(), since otherwise it could leave phantom None
on import failure.