Copy course_id into course_key and remove temporary transition code

Now that all data has been migrated to the new course_key column, we can
remove the temporary transition code.

Previous migration populates any null course_key fields, so we can now
change the column to not allow NULL values.

We make the deprecated course_id nullable, to be able to start ignoring
it later.

The migration copies content of the Answer.course_id column into the new
extended Answer.course_key column.

It skips Answer records that already have the course_key column set.

Updates README to note tag compatibility with edx-platform

releases, and adds Open edX installation instructions.

Use django Q objects instead of raw SQL query

YONK-476: fixed review step not showing tip and result icon

Add documentation about Completion block.

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.