BUG Groups can't be listed in TreeDropdownField without SecurityAdmin access

Example use case:
As a content editor I want to be able to select from a list of groups for a specific page so that they can be notified on updates.

Also see https://github.com/silverstripe-australia/advancedworkflow/issues/170
This changes the check of whether `$member` is valid, so it's not consistent with `canDelete()` etc in this class. Was this intentional?