Set up timer to log user out when session expires

This addresses item 1. of #372 (closed) fully. The app starts off a timer to log out the user one minute before the expiration of their session. This means the user never sees a stale, unresponsive UI, even after having left the app open for two hours (the current session expiry time).

Next step will be to implement a stronger link between the user's SSO session and their ICAT session, in order (notably) to automatically and silently reconnect the user to ICAT if they're still logged in to SSO.