Feature/2122 exam list page

This is for #2122-Exam list page:

* There is now a "Tests" navigation link to show the page to list available tests.
* Tests are shown in a table sorted by title accessible only to the admin users.
* Teachers or admin users can click on the "Enable Exam Mode" button to set the test in exam-mode.  Click on the "Disable Exam Mode" to disable.  Only one test can be set.
* Students will be redirected to the test exam page upon login.
* Updated the middleware to ignore urls for `/test/*`, `/static/*`, `/handlebars/*`.

Some TODOs which may deserve an issue report of their own.
* During exam-mode, the test page still shows the "Practicing" text like: "Practicing 1-digit subtraction" as header title text.
* During exam-mode, the test page highlights the "Practice" navigation link.
* Anyone can type in the test url and go to the exam page for the test, whether in exam-mode or not.  How do we prevent this?   Prevent the test page from loading if Settings.EXAM_MODE_ON is not set?

/cc @aronasorman