The idea is based on capa <clarification> nodes: https://github.com/edx/edx-platform/pull/6679

I decided to go with <span class="pb-clarification> instead of a custom <clarification>
node because supporting clarifications inside the html child block was a requirement,
but the html block is defined inside edx-platform and we aren't able to preprocess the contents
of the html block from within problem-builder.
Even if we were able to get a patch for html block to support <clarification> elements upstream,
I wouldn't feel very comfortable with that idea since the html block is supposed to contain
plain html and making it recognize and preprocess custom elements wouldn't feel right.

The best way to implement this would probably be custom DOM elements (document.registerElement),
but browser support is not there yet and there are no reliable polyfills.

Export bugfixes

Extended export

Data Export block prototype

Minor tweaks to better match the Apros theme

Fix upgrade error when <message> has no 'type' specified explicitly

Merge release onto master

Test + fix an additional failure case

Allow to include an HTML header and footer in reports.

Fixed submit button being enabled after page reload

Merging edx-release into master

Conflicts:
	problem_builder/mentoring.py
	problem_builder/questionnaire.py
	problem_builder/tests/integration/test_dashboard.py
	problem_builder/tests/integration/xml/assessment_2.xml
	problem_builder/tests/integration/xml/mcq_with_html_choices.xml
	problem_builder/tests/integration/xml/mrq_with_comments_1.xml
	problem_builder/tests/integration/xml/mrq_with_html_choices.xml
	problem_builder/tests/integration/xml_templates/assessment.xml
	problem_builder/v1/tests/xml/v1_upgrade_a_new.xml
	problem_builder/v1/tests/xml/v1_upgrade_b_new.xml
	run_tests.py
	test_requirements.txt