feat: prevent styling when using unstyled-editor class

[rpenido]

Description

This PR adds a rule that prevents course-unit-mfe-iframe-bundle.scss from being applied to components within elements with the .unstyled-editor class.

This also includes a small fix to prevent adding the background-image: none; to the select style.

• Which edX user roles will this change impact?
  "Developer"

Supporting information

• More info at https://gitlab.com/opencraft/dev/exemplar-blocks/-/merge_requests/7#note_3229990394

Testing instructions

You will need the Cover xblock to test this change.

• Checkout https://gitlab.com/opencraft/dev/exemplar-blocks/-/tree/rpenido/fal-4347/fix-styles?ref_type=heads as xblock-exemplar
• Mount it in your tutor stack tutor mounts add xblock-exemplar (as the name starts with xblock, it will be automatically installed
• Rebuild the images and relaunch the instace tutor images build openedx-dev; tutor dev launch -I;
• Enable the "cover" block in a course and add it to a Unit

Then, to test the change:

• Checkout this PR
• Run tutor dev exec cms npm run build-dev to make sure you have all assets updated.
• Open the Cover xblock editor
• Check that the Variant select is rendered with the arrow

Deadline

"None"

[openedx-webhooks]

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