feat: enable authoring MFE markdown editor waffle flag for everyone by default

[Anas12091101]

Description

This PR replaces the contentstore.use_react_markdown_editor flag with contentstore.disable_react_markdown_editor to enable the Markdown editor by default for all courses. The editor will now be disabled only if the contentstore.disable_react_markdown_editor flag is explicitly turned on.

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Relevant PRs: #36512, openedx/frontend-app-authoring#1805

Testing instructions

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• Visit http://{LMS_BASE_URL}/api/contentstore/v1/course_waffle_flags/{course_id} in your browser and confirm that the use_react_markdown_editor value is True.
• Add a Waffle Flag Course Override for flag contentstore.disable_react_markdown_editor for a course and Force On` the override choice.
• Visit http://{LMS_BASE_URL}/api/contentstore/v1/course_waffle_flags/{course_id} in your browser and confirm that the flag’s value is False.

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[Anas12091101]

@kdmccormick Could you please take a look at this PR when you get a chance?

[kdmccormick]

A couple minor code comments, looks good otherwise.

However, I am hesitant to release the Markdown editor as default-on without having performed acceptance testing yet. Could we wait until some initial testing is performed, and then make a judgement call about when to merge this PR?

FYI @pdpinch @jmakowski1123

[kdmccormick]

Suggested change

	Method to get the use_react_markdown_editor switch
	Determine whether the new markdown editor should be enabled in the react problem editor

Suggested rewording, which avoid referencing the outdated markdown editor switch

[kdmccormick]

Why remove these details?

when creating or editing problems

The markdown editor is only for problems and I think that we should make the docs clear about this.

[kdmccormick]

Suggested change

	# .. toggle_use_cases: opt_in
	# .. toggle_use_cases: opt_out

[pdpinch]

I think this is ready to be merged after a rebase. @kdmccormick let me know if you disagree.

@Anas12091101 I think we'd like a backport to the realease/teak.master branch as well.

[pdpinch]

@Anas12091101 can you rebase this?

@kdmccormick do you feel this is blocked by openedx/frontend-app-authoring#1920 or can we consider that an enhancement?