Add fulfillment option to group wishlist items by group owner

[Encotric]

As a group owner, I want to be able to click on an item on my group's wishlist and press a button to fulfill this wish. Once the button is pressed, the item should be available to all members of the group (and the waiting list, see #235).

FYI: group owners must complete the item's attributes by themselves before fulfilling the wish. Therefore, group owners can edit all wishes on the wishlist (see also #232)

Dependencies

Depends on:

• Add group wishlists #232

Acceptance Criteria

• group owners can fulfill wishes through a button on the item's details page (only visible for items that are on a wishlist + for the group owner)
• the item will be marked as available or reserved (only if users are on the waiting list) for all group members after fulfilling the wish