secure-cookies: Allow to use secure HTTPS cookies (Django setting)

Cf https://docs.djangoproject.com/en/1.4/ref/settings/#std:setting-SESSION_COOKIE_SECURE
Make code more robust to bad input (for case where DB was populated with string None)
Increase randomness of course name in testcase
update discussion blackout setting warning (TNL-442)
Pinned thread should come first when sorting

Pinned threads come on the top when we sort threads
by most recent activities but in the most activities
and most vote case it doesn't come on top. Pinned
thread should come on top in any case of sorting

TNL-90
Disable http range requests in pdf.js.

A bug in pdf.js or Chrome is causing pdf books to not load.

TNL-488
Show cohorts on the new instructor dashboard

TNL-161

Remove placeholder text
LMS: revises membership UI layout on instructor dashboard
LMS: adds static cohort group management UI to the Instructor Dashboard
TNL-160 Default cohorts, automatically created.
correcting needed markup for cohort management static UX template
addressing FED feedback
TNL-328 Server-side work for displaying type of cohort.
TNL-329 Client-side work for displaying cohort description.
Adding static rendering for cohort group creation UI

* adding styling/markup for "add new group" controls, messages, and form
* small FED and UI changes to overall cohort management UI
adding copy revisions and light styling support for cohort management
Support adding students to a cohort via the instructor dashboard.

TNL-163
converting instructor dashboard message UI to use .is-hidden vs.is-shown stateful CSS classes
LMS: revising .is-hidden utility class to be used at a global level
resolving display issues by re-implementing discussions-scoped is-hidden classes