Simple reservation / booking feature

On Cosum v0.9.17 the user has the option for "Leihanfrage" to ask the owner for an item on a defined time. It should exist a flag (similar to "faellig" and "verliehen"-flag) on the item-URL, that other users see that an item is reserved for a defined time. The owner accept reservation times on an item (time A to time B) and extern users see it, because of a flag and a button in the description of the item. https://berlin.cosum.de/gegenstaende/394