invite user from the alumni group

[stolpeo]

Currently users are invited via email because we assume that they do not have a cluster account yet. However, users can now be removed from a group and will be put into the alumni group and are therefore available again to be attached to a group. Extend the invitation interface and functionality such that an already existing user can be invited.

• Create a dropdown with alumni users. This might be a data leak because a requester can see all alumnis.
• Keep the invite via email and distinguish internally whether there is an cluster account already with a disabled user. Might be the easiest and most straight forward option.