Add form login option

[GabrielMajeri]

When creating a new Form, we'd want to allow users to specify if the forms are publicly/anonymously available on the internet, or if the users have to login and certify their identity before filling it in.

We could allow the flow creator to specify which authentication provider they want to use: e-mail-based authentication, Google, Facebook, MS365, eIDAS, eduGAIN etc. The form will then only be displayed to people who go through the indicated authentication challenge.