Inform joining user, that this room does not exist before clicking join button

[NearW]

Description

As a user, I want to already know before clicking the join button, if the room I'm about to join exists or not, so that I don't waste any time and could create the room instead.

Acceptance Criteria

• Entering a url with a non-existing room will show a dialog, that this room does not exists
• Dialog contains a button that lets the user create a new session and will redirect the user to /retro or /poker accordingly
• Dialog text: This session no longer exists.
• Button text: Create Session
• Double check if the session exists before joining the room nontheless