Persist and display history of property value changes

[jeffreymyers]

Retain a history of property value changes in the database, and provide an option to display the history for a given property. Would be nice to have some record of who changed it (at least an IP), when, and what the previous value was. Also would be good to have a single-click option to revert a property back to a previous value from the history display.

Bonus points for having an separate UI page for displaying a changelog over all properties managed by Conqueso.