Fix markdown conversion for assessments

[rtibbles]

Summary

Description of the change(s) you made

• Removes unnecessary reordering of HTML to MD conversion that was causing MD to be unnecessarily escaped
• Adds regression test
• Removes regression caused by applying 'editing' to all custom components
• Sets 'editing' attribute only in the MarkdownEditor component

Manual verification steps performed

1. Add an image to a question

2. Publish the channel

3. Confirm the image is in the exercise (on a local machine can search your .minio_data folder for the perseus file and confirm that the archive contains the image

4. Confirm that the Image context menu only appears when the markdown is being displayed in edit mode.

5. Confirm that it does not appear when the markdown is displayed in view only.

[bjester]

LGTM!

[radinamatic]

Images are fully present on Kolibri and can now be successfully removed from the exercise on Studio! 👏🏽