Add audit logging using simple-history package

[ian-ross]

Fixes #244.

Note that this is likely to have a substantial performance impact, because it logs all changes to all models into separate history tables in the database. This is required to guarantee that we can reconstruct all the history of database entities in the future, but it will have an impact.

Also note that any new models created from now on will need to have a history field added in the same way as is done for the existing models in this PR.

[wonderchook]

@amplifi something to be aware of related to performance. Later we are going to add more intelligent ways to track object history, but this is a temporary solution (temporary as in at least 3 months).