Fixed incorrect theme dir order for media files

For jinja2.FileSystemLoader(config['theme_dir']) it is correct to specify own and base theme directory in this order.
For copy_media_files this order has the problem that a file (css/image/...) which exists in both own and base directory is copied from the own theme first and overwritten by the file from the base theme later.
Could you add a comment here, something along the lines of "Reversed as we want to copy from the builtin themes first and then overwrite with the custom themes"

I think this is the right solution to the issue, but I also think it is a bit unintuative. Otherwise happy to merge, thanks!