Folder behaviour improvements

- [x] When creating a new folder, the current user should not automatically be added as a manager when at least 1 other manager has been specified. When no managers have been specified, the current user should be added as a manager
- [x] The rules around adding a private item to a folder are currently too restrictive. Therefore, the rules should be modified to allow a private item you don't manage to be added to a folder you can add items to when the private item is part of a group the current user is a member/manager of as well.