"Session Expired" dialogue should replace "No recent activity" dialogue, doesn't behave well with SSO.

Steps to reproduce:

1. Set $CFG->sessionexpiry to 5 minutes
2. Wait on a page until the "No recent activity" dialogue is displayed.
3. Continue to wait until the 5 minutes are up
4. Click on "Extend session"
   1. Expected: Your session is extended
   2. Actual: Your session cannot be extended and a dialogue with the message "Your session has expired, please login to continue" message is displayed.

As an added strangeness, becuase we are using an SSO system, when you click "OK" on the "Session expired" dialogue and click a link to a new page, our auth plugin gives you a new Moodle session without logging in, despite you being told you need to.

An improved user journey would be:

1. Wait on page until the "No recent activity" dialogue is displayed.
2. Continue to wait until the 5 minutes are up.
3. Replace "No recent activity" with the the "Session expired" dialogue, with a message like "Your session has expired, please reload the page to continue", and a "Reload page" button.  The user can now reload the page (with a new session if using SSO, or redirecting to login if not), or close the dialogue if they want to copy/paste something they've entered on the page and reload when they are ready.