Fix Style/MixinUsage RuboCop offenses
Fix instances of: https://docs.rubocop.org/rubocop/cops_style.html#stylemixinusage in this project.
The method I'm using here is to prefer use_helpers
(documented at: https://nanoc.app/doc/helpers/) instead of include
. I'm sure there are other ways to resolve the RuboCop offenses.
Edited by Evan Read