Migration command to move course content from GridFS to openedx_content [FC-0138] #39258

Depends on openedx/openedx-core#890 and #39256.

Implement a migration command as described in openedx/openedx-core#880

Once migrated, a course's "Files" page and other usages of static assets (e.g. embedded in XBlocks, or the course about image) should be working and served from openedx_content only.

Private ref FAL-4431