Integrate group functionality for the lend process

[NinaMohnke]

User stories

The owner of items are not be users, but also groups e.g. Fachgebiete. So when dealing with the owner of an item, when e.g. returning items to the owner or sending notification to the owner. It should also be possible to do it to a group, rather than a user.

Dependencies

Depends on:

• Create User Grouping System #8

UI Mockup

Acceptance Criteria

• Items should be returned to all owners in a group, when the owner is a group
• Waitlist should be updatet to also check for groups and not only users
• Notifications should be sendable to a group
• When one User of a group accepts the request, all other users should have their respective request message labled as read