[DO NOT MERGE] Custom paper_trail YAML serializer for JobReady

paper_trail's YAML serializer breaks with carrierwave-cascade. This is a custom serializer to work around this issue, by special-casing carrierwave file attrs to store just the URL.
`extend self` is an idiom for making module methods callable on the module itself, e.g.:

```ruby
2.0.0-p247 :003 > module Foo
2.0.0-p247 :004?>   extend self
2.0.0-p247 :005?>   def bar
2.0.0-p247 :006?>     puts "in bar"
2.0.0-p247 :007?>     end
2.0.0-p247 :008?>   end
 => nil 
2.0.0-p247 :009 > Foo.bar
in bar```