[WIP] Refactor MAGE integration

Refactors the MAGE integration by
* introducing a `Bookkeeper` factory
* introducing `Mage::Bookkeeper` and `Fake::Bookkeeper` implementing the interface of `.new(purchase)` and `#keep`
* adding a Rake task to export the current MAGE mappings
* removing MAGE-related columns from the database
* adding a controller and routes to update mappings on disk

Please note that this is still a work in progress! Some thing left to do include:
* [ ] make sure the mappings stay in memory most of the time, since they're not updated that often
* [ ] make sure the mappings are reloaded properly after updating them
* [ ] add authentication to the mappings controller