Use nodeenv to manage node version in Jenkins builds

Add date_passed column to PersistentCourseGrade model

Fallback to default ZENDESK_URL when not in JSON

The column is populated with the current timestamp when a passing grade
is persisted for the first time.  "Passing grade" is defined as having a
non-blank value for letter_grade.

TNL-5888

param typo

This change:
* gives us the flexibility of managing node from pull-request to pull-request. It'll be a huge lift to folks that want to test out node upgrades like @bjacobel or @andy-armstrong
* Sets us up for a port to Xenial, which will be accomplished in part by using nodeenv for managing the node environment. See a companion PR on the configuration repo for edxapp: edx/configuration#3444

This pull request does not:
* Attempt to solve any stability issues in builds related to node.
* Make node installs any faster
* Create any caching infrastructure to speed up node installs. They will be approximately as fast as they were before. Downloading the node bits build-to-build adds about 3 seconds, which is worth it for the added flexibility.

[SOL-2126] Verification status endpoint

Fix discussion user roles

Single search request to fetch notes per unit

fix inconsistent capitalization in education drop down

ECOM-2470 & SOL-2126

Wl-613 | Removing the example theme.

Teams tab inline discussion new post form text wrap

Update translations (autogenerated message)

PLAT-1074 Add support for django-user-tasks

WL-798 Allow for hiding course mode banners on student dashboard

TNL-5621 Fix discussion topics in MIT course.

TNL-5721 – Fix check source module access before appending it to required modules

Upgrade Pillow CVE-2016-9190 CVE-2016-9189

Optimize async grades computation

TNL-5909

Release 2016 11 08 conflict

Refactor discussion tab UI code into parts to be reusable for inline discussions TNL-5669

TNL-4163

Update course grade query parameters and check scores when updating subsection grade

Update to UXPL 0.17 (grids compilation performance work)

Move embargo from common to openedx/core/djangoapps.