Enhancement: Prevent migration between normal and guest users in UI

OC 10 does not support the migration from normal users to guest users and

vice versa. The fix clarifies this behavior in the Web UI by omitting the

guest-group when adding or editing (non-guest) users and by disabling the

ability to remove guest users from the guest group.

https://github.com/owncloud/core/pull/38298

https://github.com/owncloud/guests/issues/431