Group managers should not be allowed to send invitations to users that are already part of the group

Problem/Motivation

When group managers are inviting users to the group they currently can select existing members too. The invite batch will go through but no invites will actually be sent, but this is still a bad UX.

Steps to reproduce

1. Create a group
2. Invite a user to the group and accept the invitation as that user
3. After the user is already a member of the group invite him again.
4. See that no error will show up

Proposed resolution

If the user is already a member of the group an error should show up.

Remaining tasks

User interface changes

API changes

Data model changes