Creates audit log for artifact transitions

Audit log for artifacts tracks the transitions between states.
Added sponsorship and potential sponsorship logic for artifacts, based
on the transition logs.
Configures admin interface to show all the transition data for artifacts
and sponsorship data.

Upon further consideration though, we won't want to use the logs to key
off for the sponsorship data - because users will be able to withdraw
requests. Tomorrow I'll create a separate sponsors table instead, as
well as provide test coverage for these features.