Allow user to select preferred slots prior to the registration phase

[Stefan Bley]

In order to create the best possible schedule, the organization team needs some rough information about how many users are likely to register for a slot. To gather that information the app should display a view that shows all slots, unscheduled, in a list and allows the user to mark a couple of them as preferred slots (up to 10).
The view should not be displayed after the scheduling has finished and the registration phase has started.