Add optional information on /hub/home (or header?) website

Currently, whenever there's an update of the hpc information of a user, we have a cronjob which will update Unity + JupyterHub. Therefore, the user can use this new information without a re-login.

When we move to the Unity (JSC-Login), the update of Unity will not be possible anymore for us. Unity will only be updated when a user signs in. That's why we need an information screen for the user, like "Hey your access to the hpc-systems was updated. Click to update".

This information should be shown, if there's a flag in the user's auth_state. Like if auth_state.get("reauthenticate", False): ... . The user would have to click on a button, which will redirect them to https://....fz-juelich.de/hub/reauthenticate . The user will be logged out automatically at this url and forwarded to the website, where they must enter their credentials.

@petrova1 Could you implement the frontend part of this? I don't if it's best put into the header or somewhere in /hub/home. Maybe you have better ideas for it.