Adds a new upload field for a second dark mode category logo.
This alternative will be used when the browser is in dark mode (similar to the global site setting for a dark logo).
* File.exists? is deprecated and removed in Ruby 3.2 in favor of
File.exist?
* Dir.exists? is deprecated and removed in Ruby 3.2 in favor of
Dir.exist?
Uploads can be reused between site settings. This change allows the same
upload to be exported only once and then the same file is reused. The
same applies to import.
