Convert existing users to guest users

[bentuna]

It would be very helpful to "downgrade" existing "full users" to guest users.

I tried to achieve that by adding a user to the Guests group, but that results in a infinitely spinning "loading" animation in the group dropdown and nothing happens.

[sebastiaanveld]

Would like to have this too; we had to create some external users as full users to be able to allow them to use OTP for protection. At some point OTP hopefully #149 is solved, so it would be nice to be able to convert those users to guest so the view ("Limit guest account to an app's allowlist" setting) within NC gets limited.