Fix flaky library test TE-745

Updated version of PyContracts.

split unittests into shards for in jenkins

Finish Country Access (Part 2 of 3)

Avoid logging errors when the image path is not provided for invoice PDFs

fix unicode error on cms settings page

ALT text describes image, not action

Update translations (autogenerated message)

Ignore staff member's cohort when masquerade is on

ECOM-1028 adding linked-in button , config model and migration for linke...

Discussion forum should be independent of mathjax 

Work around a Youtube API bug that causes Safari to ignore the stored speed

TNL-1269

ECOM-1028 update the migration and move the code into class method.

At LMS dashboard course listing the course cover has alt
attribute that have course name and then Cover Page it describe
image not the action which is to take user to course home page

TNL-1383

Mobile performance improvements

Release

Add our custom pylint plugin

Updated _get_courses_with_access_type

Fix grades download in courses with cohorted content

get the raw DescriptorSystem, not the CombinedSystem when instantiating ...

The overall behavior of the __init__ method has remained the same.
What's changed is how it determines whether a field is explicitly set.

Instead of using the slower performing editable_metadata_fields, it
calls _field_data.has directly to check for explicitly set fields.

Block users from enrolling in a course if the user
is blocked by country access rules.

1) Enrollment via the login/registration page.
2) Enrollment from the marketing iframe (via student.views.change_enrollment)
3) Enrollment using 100% redeem codes.
4) Enrollment via upgrade.

This does NOT cover enrollment through third party authentication,
which is sufficiently complex to deserve its own commit.

Finish Country Access (Part 1 of 3)

add logging for send_email_task

Courseware search: Change name of results page from "Searching [course n...

Reorganise reindexing operation

An upgrade to verified should emit a mode_changed event.

Add history table for course access rule changes.

Provide test utility for simulating restricted access.

Provide `redirect_if_blocked` method for integration with other
parts of the system (will be used for blocking enrollment).

Add info-level logging explaining when and why users are blocked.

get the raw DescriptorSystem, not the CombinedSystem when instantiating xmodules (TNL-1226)

quality fixes

remove trailing comma

This change will also move us to bok-choy instead of lettuce for
these scenarios, and re-enable them. See TE-736.

Includes some refactoring of repeated event-checking code as well.