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)
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
Quiet stevedore.extension logging in bok-choy tests
Don't put all of xmodule_js as src (changes coverage metrics).
Four Unicode fixes in discussion
Add new enrollment message to the dashboard

The body of the enrollment message template

Tokenize platform name in message.

Changing to a datetime enrollment approach

Adding sorting. A little refactoring.

Adding confguration model for time delta

Adding admin registration and basic form for new config model.

Fixing docstring typo

Updating default time delta to 0, adding test to show it disabled functionality.

Removing the form from configuration and tweaking the enrollment message html
Merge pull request #5424 from edx/adam/change-discussion-blackout-wording

update discussion blackout setting warning (TNL-442)
Merge pull request #5436 from Edraak/edraak/omarithawi/fix-four-unicode-issues-in-discussion

Four Unicode fixes in discussion
Merge pull request #5459 from edx/zoldak/quiet-stevedore

Quiet stevedore.extension logging in bok-choy tests
Merge pull request #5420 from edx/sanchez/enrollment-success-message

Add new enrollment message to the dashboard
Merge pull request #5366 from edx/usman/tnl367-transaction-level-decorator

MySQL transaction isolation level decorator.
Update final text strings & readthedoc URL

TNL-401
Use ddt for shib tests
Remove test cleanups

Since `ddt` should make them unnecessary
Address PR review comments