Delete project codes originating from lotsofcake that were deleted server-side

[frankosterfeld]

Tasks should get a "source" property that allows to store whether a task was synced from, say, lotsofcake or created manually. It should contain the lotofcake URL for tasks imported from there.

When syncing with lotsofcake, tasks originating from lotsofcake that are not in the project codes XML anymore should be deleted. If there are unuploaded events using the project code, the code should be kept and the user should be asked to fix the issue.

[frankosterfeld]

Not sure that's a good idea, we wouldn't want to delete tasks with local events associated. Better would be to have them in a "deleted" state similar to expired which prevents them from being selected for new events. Tasks not used for events could be deleted though.