New hflossk/resources links

This allows the hflossk websites to use links specified in oer.yaml. Which will be at the app_root directory of each child website. This corresponds with https://github.com/ryansb/hfoss-site/pull/1

* [x] Allow Links to be Added to oer.yaml that will appear in the resources section
* [x] Allow for directories in the static folder to be empty and have a subsection consist only of links

This would fix the issue brought up in https://github.com/decause/hflossk/pull/425
I'm actually not a fan of making this an option for books. I'd like to (sort of) force folks to include course materials like books because file hosting sites can be pretty unreliable.