when donating to the Museum, prefer to donate items which were made by you, and allow re-donating an item if you can "upgrade" to an item made by you

the endpoint in `GetDonatableItemsController` is responsible for getting items to donate; I'm pretty sure all the changes can be done there.

(side-note: the front-end should be updated to show an item's maker, but that's a different project. if the front-end isn't open source yet, just bug me (Ben) to make that change.)