I have no idea how to use the editor form control in the new blocks/html/edit_form.php I have just made.

At the moment this is used from blocks/edit.php, but that code may have to move to lib/blocklib.php.

There may be other blocks with HTML content, I am not sure yet.

By the way, people definitely like putting images in HTML blocks so we have to fix this. At least all blocks have a context now.