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

Enable freeform answer submit when feedback is hidden

When problem builder is configured to hide feedback upon revisiting a question
(pb_hide_feedback_if_attempts_remain option is enabled), when the user returns
to a problem builder block which contains only a freeform question,
the user is not able to click "Submit" without changing the answer text first.

This patch fixes that by keeping submit button enabled if
pb_hide_feedback_if_attempts_remain is turned on.

MCKIN-3890

StepBuilder: Fix inconsistent feedback for MCQs

When the pb_mcq_hide_previous_answer is set to True, MRQs should behave
the same way as MCQs behave - previous choices should not be displayed
and the checkmarks should be hidden.

Add instructions for enabling dashboard in studio

Use dashboard image from master branch in tests

So that the omar/report-download branch can be deleted

Fix dashboard report image download

Fix dashboard report image download (edx-release hotfix)