Add Auditing

[flwyd]

Changes to important data like people and schedules should be audited. It's probably easiest to audit all models. Write-only is fine if the audit trail is easy to read through SQL. For bonus points, a simple audit view which shows changes by a user or changes related to a person would be nice as long as it doesn't take too much effort.

https://github.com/nearinfinity/auditor and https://github.com/collectiveidea/audited are two similar gems that handle auditing. There may be others, too.
Of particular interest is the ability to associate an audit entry with a model further up the association chain. For instance, "work log 57 was deleted" isn't very helpful, but "work log 57, which was associated with person 22 was deleted" is more useful in trying to understand why someone's hours don't match what they expected. Note that the association chain in this example is work_log->participant->person.