change html_editor's upload_path to file_uploads_path

1. Now students cannot upload images to forum due to permission issues.

2. I changed html_editor's upload url to 'file_upload_path' instead of 'course_file_upload_path', thus users will be able to upload images through html editor.

BTW, To make sure that I'm doing the correct thing, I think course_file_uploads_path is for materials( not images)?  for this path we will check permissions.