This adds a migration to turn on the new React-based Markdown editing option
for newly-created ProblemBlocks for all authors. The option is nested
under "Advanced settings", just like the Advanced (OLX) problem editor.
The migration is written such that it will *not* overwrite an existing global
"No" waffle flag. Furthermore, our waffle utilities are designed such that
org-level and course-level overrides will still take preference over the flag
that the migration creates. Therefore, this should only take effect in cases
where no existing waffle flag applies.
BREAKING CHANGE: Operators who do not want the new React-based Markdown editor
to be shown should create a flag contentstore.use_react_markdown_editor with
the value "No".
We plan to backport this to Teak.
Prior art for flipping waffles like this: 9b0024376a
Part of: https://github.com/openedx/platform-roadmap/issues/384
Co-authored-by: Muhammad Anas <muhammad.anas@arbisoft.com>
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