Student re-login during an LDB quiz session

This patch makes it possible for students who get logged out while
taking a quiz in the LDB to re-login and resume their quiz with no
interruption.

fixes CNVS-9986

TEST PLAN
---- ----

  - Create an LDB quiz and start taking it in the LDB browser.
  - Use a different browser, or computer, to login using the same
    student account, and then log-out
  - Try to modify your answer in the LDB, and:
    - verify that you are prompted with a login pop-up, and:
      - proceed to test cases

> Case 1: Successfully logging in

  - when prompted, enter the correct credentials, and verify that:
    - the pop-up automatically closes itself
    - you see a successful status message in green
    - you are able to modify your answers and continue taking the quiz
      normally
    - you are able to submit the quiz

> Case 2: Failing to log in

  - when prompted, enter bad credentials, and verify that:
    - the pop-up does not close
    - you see an erratic status message in red

> Case 3: Logging in as someone else

  - when prompted, use a different account to login, and verify that:
    - the pop-up closes itself
    - the pop-up immediately re-opens itself
    - you see the erratic message of authentication failure

> Case 4: Trying to escape

  - when prompted, try the following things:
    - close the pop-up manually by clicking the X or close button
    - click on stuff in the background (quiz window)
  - verify that:
    - when the pop-up loses focus, it automatically "pops" itself back
      onto the foreground
    - you can not modify answers by clicking the background, like
      clicking a radio button or a checkbox
    - you can't do *anything* except for logging in

DEMO
----

http://amireh.net/Funky-LDB-Login.avi

Change-Id: I0edd993b388ab73fe4a4d8fa3f96b7422be7ceb1
Reviewed-on: https://gerrit.instructure.com/28874
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Ryan Florence <ryanf@instructure.com>
QA-Review: Myller de Araujo <myller@instructure.com>
Product-Review: Ahmad Amireh <ahmad@instructure.com>