Avoid count(*) query in management command.

Count(*) is very slow on large tables.

Amended headings to ensure accessibility.

* Amended templates - h2 elements to h3, h3 to h4 and associated CSS.
* Bumped version to 2.6.8

Add management command to manually migrate course_id field.

Normally the value of deprecated Answer.course_id will be copied into
the new Answer.course_key field by a migration, but if your answer table
is very large, you might want to use this management command to perform
the copying in batches instead.

Override layout of choice label in Apros theme

Do not show completion checkmark when hide_feedback is enabled

This makes the Completion block consistent with the Answer block.

When 'pb_hide_feedback_if_attempts_remain' is set to True in
XBLOCK_SETTINGS, once the user submits an answer to an Answer (freeform
long answer), the green checkmark is not visible after refreshing the
page, even though the given answer/text is still visible.

This commit makes Completion blocks consistent with that behaviour.

Fix display of MCQ/MRQ choices with long text.

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