Enabling NativeAuthenticator removes all logins including admins

[MikkoFinellEdupower]

Bug description

After Enabling the authenticator no users can log in anymore. My admins username and password results in "invalid username or password" and new users cannot login either because they would have to be approved by the admin apparently, who cannot login anymore.

Expected behaviour

I expected new users to be able to register, and I expected the pre-existing admin accounts to still be able to login like before and to approve users from the control panel.

Actual behaviour

Admins or any pre existing users can not login any more.

How to reproduce

1. Follow the install guide for Digital Ocean.
2. (Optional) Enabled HTTPs
3. Logged in with my original admin user, created the password at the login screen.
4. (Optional) create some users, admin or otherwise, from the control panel.
5. (Optional) I logged in and out multiple times with admin and other users to make sure 100% I had the passwords stored correctly in a password manager.
6. Enable user signup. https://tljh.jupyter.org/en/latest/howto/auth/nativeauth.html#enabling-the-authenticator
7. None of the users can login anymore now.

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[mtav]

You should be able to fix this by:

• adding c.NativeAuthenticator.import_from_firstuse = True in a configuration file in /opt/tljh/config/jupyterhub_config.d/

• or ideally by using: sudo tljh-config set auth.NativeAuthenticator.import_from_firstuse True

Unfortunately, it does not work properly at the moment. There are two problems:

1. tljh-config does not work properly to configure NativeAuthenticator (Can't get NativeAuthenticator to work #295) do to a problem with setting sets/tuples/lists/etc (Using tljh-config set for properties which require tuples, lists, dicts etc. #709, tljh-config set with a list as value  #549) -> See workaround suggested here.
2. NativeAuthenticator does not properly import the FirstUseAuthenticator database. -> I submitted a fix for it here.

Central issue where this seems to all be worked on: #264