feat!: enable authoring MFE markdown editor waffle for everyone by default (#36872)

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>

cms/djangoapps/contentstore/migrations/0011_enable_markdown_editor_flag_by_default.py

@@ -0,0 +1,25 @@
+from django.db import migrations
+
+from cms.djangoapps.contentstore.toggles import (
+    ENABLE_REACT_MARKDOWN_EDITOR
+)
+
+
+def create_flag(apps, schema_editor):
+    Flag = apps.get_model('waffle', 'Flag')
+    Flag.objects.get_or_create(
+        name=ENABLE_REACT_MARKDOWN_EDITOR.name, defaults={'everyone': True}
+    )
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ('contentstore', '0010_container_link_models'),
+        ('waffle', '0001_initial'),
+    ]
+
+    operations = [
+        # Do not remove the flags for rollback.  We don't want to lose originals if
+        # they already existed, and it won't hurt if they are created.
+        migrations.RunPython(create_flag, reverse_code=migrations.RunPython.noop),
+    ]