Download current item file

There should be an option in the admin interface somewhere to turn this off. Defaulted to off, actually.

I don't think we really need a permission for this. My chief thing is that I don't want a future update to cause changes to people's existing galleries. That's why this should be optional and turned off by default. A global option is probably sufficient for this.

I still don't think the permission is really all that necessary, but if you add it, make sure to add some instructions on the global enable/disable option to point people to the permission. Otherwise, people will turn it on and then be confused why it doesn't work.

Thanks for your work on this patch. There are few issues with it before I can commit:

1. Code Style: Please follow the Drupal code standards: https://www.drupal.org/coding-standards.
2. Support for no media file: It's possible to have a node gallery without a media field specified. Imagine a gallery where people are posting Youtube embeds or something. It's possible. I think your node_gallery_api_get_cur_item_media() function is going to throw errors in that case. I didn't test it, but it assumes that $file exists when it gets the $file->uri.
3. In node_gallery_api_get_item_navigator(), we shouldn't build the full sized image path unless this option has been turned on.
4. Ideally, when you clicked Download, it should actually download rather than open in your browser. This is possible if you use a content disposition header, but it's a significant amount of extra work, because you won't be able to link directly to the file like you do now. You would have to build a menu route like node/%node/download and have that handle the header and passing the file through. I'll accept the patch without this change, but just throwing it out there for you to consider.