Add paper trail to models

@danmaclean This PR adds PaperTrail to Organisms, Genomes, Experiments and Features as a start. From what I can tell this will be what you need from an audit perspective.

However, I'm not 100% au fait with the data model at the moment. The diagram in manual.pdf doesn't appear to be accurate (for example an Experiment appears to belong to a Genome directly, not via a Reference). If you can outline how the data model works I can review further to see whether we need to extend PaperTrail to Reference, Sequence, Predecessor, Parent.