Resolve "Popup on expired session"

What does this mr do?

Notifies the user when his session has expired as requested in #338 (closed).

Screenshots

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• I've made a listener for the destruction of HttpSessions.This triggers a websocket event to the user which then renders a modal with the message.
• I've moved HttpSessionConfig and the new listener class to their own session package.
• I'm not very happy with the changes I made to global.js, but I couldn't think of a better way to listen for such events. Currently websockets aren't hooked into HttpSession so they don't close when the session expires, although that may be a good thing it may be unintended. With this MR the socket is closed after the user receives the session expired notification.

Does this MR meet the acceptance criteria?

• I have added a changelog entry to reflect the significant changes I made.
• Tests were created to test the feature.
• I have updated the documentation accordingly.
• I adhere to the style guide.