Native Authenticator not working after configuration

[piskernik]

Bug description

In Issue 295 a workaround was stated, to bring Native Authenticator to work, however som critical additional information is missing

How to reproduce

apt-get update && apt-get upgrade
apt install python3 python3-dev git curl
curl -L https://tljh.jupyter.org/bootstrap.py | sudo -E python3 - --admin adminusername
sudo tljh-config set auth.type nativeauthenticator.NativeAuthenticator
sudo tljh-config set auth.NativeAuthenticator.admin_users adminusername
sudo tljh-config set auth.NativeAuthenticator.allow_all True
sudo tljh-config reload

This will not allow login of the adminusername.

Work-around

In contrast you need to do the following:

apt-get update && apt-get upgrade
apt install python3 python3-dev git curl
curl -L https://tljh.jupyter.org/bootstrap.py | sudo -E python3 - --admin adminusername
sudo tljh-config set auth.type nativeauthenticator.NativeAuthenticator
sudo tljh-config set auth.NativeAuthenticator.admin_users otheradminusername
sudo tljh-config set auth.NativeAuthenticator.allow_all True

Then cd /opt/tljh/config/jupyterhub_config.d/ and create a new file:

nano NativeAuthenticator.py

Paste this into it:

import os, nativeauthenticator
c.JupyterHub.template_paths = [f"{os.path.dirname(nativeauthenticator.__file__)}/templates/"]

Save it with CTRL-X and Yes and then sudo tljh-config reload

Now REGISTER as new user with otheradminusername. 
You will get access.

### Expected behavior
Of course it would be good, if no workaround would be necessary. At least the workaround should be described in the documentation for native authenticator.

[piskernik]

The reason is, that the jupyterhub.sqlite database in /opt /tljh/state is not prepopulated with the existing user, when the change to Native Authenticator occurs.