To allow users or groups to be added to single work packages as contributors, we need to establish a new membership system that allows this. The current `members` table only supports adding users to whole projects.

* [ ] Create a new table that links WorkPackages with Begin of the insertionusersEnd of the insertion Begin of the deletion members (polymorphic: group or user)End of the deletion and allows assigning **one** role for that member
* [ ] Change the `visible` scope of WorkPackages to also include those, where the requested user is either able to view the WP because of project membership or direct WP membership

Begin of the insertion~~ExtendingEnd of the insertion Begin of the deletion ExtendingEnd of the deletion the Begin of the insertioncurrent~~ `~~members~~` ~~relationEnd of the insertion Begin of the deletion current `members` relationEnd of the deletion is not really an option, as it does not support polymorphism and changing that would be a really big data model change. Also, it supports assigning multiple roles to a membership, that is something that is not needed for the WP Begin of the insertionmemberships~~End of the insertion Begin of the deletion membershipsEnd of the deletion