Add a new permissions name 'visible_in_catalog' to the access.py file to...

... allow for an explicit 'show in catalog' setting so that White Label sites can show/hide courses outside of Enrollment/Start time windows
while I appreciate the desire to keep settings that aren't intrinsic to content out of `course_module.py`, this now will be mixed into all xblocks in LMS, right?  Is that the best way?  @ormsbee want to weigh in?