Introduce Answer.course_key and deprecate course_id.

This introduces a new 255-char course_key field to the Answer model.
The old 50-char course_id field is still present, but is deprecated and
will be removed in next release.

We cannot simply extend the existing course_id field because we have
some large problem_builder_answer tables in production and the migration
to extend the column would lock the table causing issues in production.

The code is updated so that it uses the new course_key column, but falls
back to course_id.
In next release, a data migration to copy course_id data to course_key
will be provided and the course_id column dropped.

Remove migration that extends course_id column

Migration locks the table, which is not desired since the production
table on edx.org is quite large.

The course_id field will be extended in a multiple steps instead.
For more info see:

- https://github.com/edx/edx-platform/pull/14013
- https://github.com/open-craft/problem-builder/pull/131

Implement Completion XBlock

Extend course_id field to 255 characters.

It stores course keys, which can be longer than 50 characters.

Standard field size for course key fields in edx-platform is 255
characters, so extend our course_id field to 255 chars.

2.6.2 Instructor Tool uses the Course Blocks REST API

* Removes the python course tree traversal logic, and replaces it with Javascript.
* Adds student_view_data() to downloadable block types to reveal the "question"
  text to the Course Blocks API

Uses the same import path for instructor_task.models as edxapp

Includes version bump to 2.6.1

Fixes RuntimeError: Conflicting 'instructortask' models in application
'instructor_task': <class
'lms.djangoapps.instructor_task.models.InstructorTask'> and <class
'instructor_task.models.InstructorTask'>.

Bump version to 2.6.0

Accessibility updates

* Moves the choice-label <input> elements inside of their <label> tags.
* Wraps long-answer and slider question text in a <label> tag.
* Uses <th> instead of <td> elements inside <thead>, for the instructor tool and plot preview tables.
* Links the feedback, and choice-tip text with the choice labels via aria-describedby.
* Wraps the choice-result feedback icons inside the label elements, to match CAPA a11y design.
* Adds translated, aria-visible labels for the "correct answer/choice" checkmark and "incorrect answer/choice" X.
* Adds the 'aria-live="polite"' attribute to the divs whose content changes dynamically.
* Removes block-level children from <legend>, placing the h3.question-title outside the <fieldset>.
* Fixes tests broken by above commits
* Adds tests for new aria-label attributes..

Handle unresolvable anonymous ids gracefully.

Remove "Message (Assessment Review)" from Problem Builder in Studio

Shows the "on-assessment-review" and "on-assessment-review-question" buttons in Problem Builder only if (deprecated) assessment mode is enabled.

* Always shows the "on-assessment-review-question" button in Step Builder Questionnaire problems.
* Fixes button styles to prevent long button names from overflowing the buttons.

Update master with changes from edx-release

Update xblock-utils to v1.0.3.

setuptools to a specific version for now.

XBlock is listed as a dependency in requirements files for xblock-sdk,
xblock-utils, and problem-builder itself. These files list different (but compatible)
versions of the XBlock repo. Because of that, XBlock gets installed multiple times
when installing dependencies for CircleCI builds.

Starting with setuptools 25.0.0, the behavior for installing packages on top of
existing installs changed; see https://github.com/pypa/setuptools/issues/729
for an example issue that this change in behavior caused.

For problem-builder CI builds, the issue was that after installing all dependencies,
the egg link for XBlock ended up pointing to the installation of problem-builder itself,
creating a situation where the code that ships with XBlock was not accessible from modules
that depend on it. This caused imports for any classes defined in the XBlock repo to fail,
and CircleCI builds errored out.

To fix this, we temporarily pin setuptools to the most recent version
that does not exhibit this behavior (24.3.1).

An upstream fix seems to be underway (via https://github.com/pypa/pip/pull/3904),
but it hasn't been merged/released yet.

New version includes fixes from https://github.com/edx/xblock-utils/pull/38 and https://github.com/edx/xblock-utils/pull/39.

Fix mako template context error

Hide MRQ question-level feedback on return.

When the pb_mcq_hide_previous_answer is set to True, question-level
feedback should not be displayed when user returns to the page.

Stopping video playback for hidden steps