647 session warning react

[erinz2020]

implement same session expire warning as jsp pages.

some points worth noting:

1. when countdown ends, the page shows user is logged out, and was prompted to login again, actually user is not logged out, the session expire warning is not tied with actual tomcat user session
2. even if the user remains active on the page, when the set time is reached, the modal will still appear if the page hasn’t been refreshed during that time. although it's unlikely for a user to stay on the same page for 20 minutes without navigating away.
3. if the user clicks the close button, the modal will not reappear when the 20 minutes are up, as long as the page isn’t refreshed.

PR fixes issue #647

[TanyaStere42]

👍