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)
update discussion blackout setting warning (TNL-442)
Disable http range requests in pdf.js.

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

TNL-488
Disabled failing test test_video_end_time_with_default_start_time.
Properly escape CourseKeys and UsageKeys in html templates

[LMS-11372]
[LMS-11370]
Created commit_on_success_with_read_committed decorator.

This decorator executes the decorated function inside a
transaction with isolation level set to READ COMMITTED.

TNL-367
Tests for the commit_on_success_with_read_committed decorator.

TNL-367
commit_on_success_with_read_committed should only work with atmost
1 level of transactions.

TNL-367
Added commit_on_success_with_read_committed decorator to views
which raise IntegrityErrors.

TNL-367
Merge pull request #5455 from edx/usman/tnl488-disable-range-requests-in-pdf-viewer

Disable range requests in pdf viewer.
Kill DISABLE_RESET_EMAIL_TEST flag

We're better served by just directly testing if we're in the LMS or not.
Translate strings in LMS Underscore templates
Quiet stevedore.extension logging in bok-choy tests
Merge pull request #5392 from cpennington/escape-course-ids-in-html

Properly escape CourseKeys and UsageKeys in html templates
Merge pull request #5461 from edx/release

Release
Merge pull request #5458 from edx/andya/lms-underscore-i18n

Translate strings in LMS Underscore templates
Merge pull request #5457 from edx/kill-DISABLE_RESET_EMAIL_TEST-flag

Kill DISABLE_RESET_EMAIL_TEST flag
Don't put all of xmodule_js as src (changes coverage metrics).
Four Unicode fixes in discussion