Implement field visibility classification

[simonaszilinskas]

Description

Currently, the registration form fields are visible in a pre-defined manner. To provide organizations with more control over the visibility of user information, we should introduce a feature that allows administrators to classify each field as either private (accessible only to administrators) or public (accessible to all users).

Proposed solution:

1. In the configuration of extra sign up fields, allow to set the field as visible or not visible;
2. Give the user a visual indication on whether the information they fill will be visible or not;