Feature flag for the "start"-page

[tibroc]

Is your feature request related to a problem? Please describe. While the current starting page looks nice and makes for a good advertising for PILOS, it quickly becomes useless and tedious for users because it doesn't serve a real purpose other than advertising.

Describe the solution you'd like Make it configurable to show this page for the PILOS-admin. If turned off, the behaviour should probably be:

• if the user is not logged in, show the login-page
• if the user is logged in, start with the room list

Describe alternatives you've considered

• Only showing this page to users that are not logged in, but this also becomes tedious once you are familiar with the system.
• Only showing the page once to every user.

Additional context I am aware of https://github.com/THM-Health/PILOS/issues/1216, but multiple users reported this. Also the discussion there shows that this page might lead to follow up requests from clients.