add a migrations to the new django_comment_common to keep things a bit more consistent with our normal useage.

update comment models.py to not have the dependency on get_course_by_id (which is in LMS). Also explicitly set the db_table name because django auto generates table prefixes according to the djangoapp name

move some files from lms/djangoapps/django_comment_client to common/djangoapps/django_comment_common