Only last login valid during "exam mode"

[cysjonathan]

This feature is meant to be broken up into incremental levels of increasing security.

1. Implement an "exam mode" that marks users as "in-an-exam" (to discuss: software design for exam mode).
   A suitable FE indicator should be displayed: "You are in an exam: Your access is limited to course EXAM"
2. During the exam, users are only allowed to have 1 active login, all past logins are marked as stale and invalidated.
3. During the exam, users are locked to the current course (cannot access other courses).
   Access denied pages with appropriate error messages should be displayed: "You are in an exam: You do not have access to course XXX until the your current exam submission is finalized"
4. During the exam, basic user activity is logged --- POST/PATCH/DELETE of comments, answers, discussion posts.
   A suitable interface will be needed to display and filter activity information as well.
5. During the exam, all user activity is logged --- POST/PATCH/DELETE of every action) (to be discussed if required).